Compiler warnings in Ubuntu build

From Ubuntu 20.04 build log:

* CMake
>CMake Deprecation Warning at libraries/fc/vendor/websocketpp/CMakeLists.txt:9 (cmake_minimum_required):
Compatibility with CMake < 2.8.12 will be removed from a future version of
CMake.
>
> Update the VERSION argument value or use a ... suffix to tell
CMake that the project does not need compatibility with older versions.

* memset
>/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/sha1.cpp: In function ‘void fc::from_variant(const fc::variant&, fc::sha1&, uint32_t)’:
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/sha1.cpp:93:38: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::sha1’; use assignment or value-initialization instead [-Wclass-memaccess]
93 | memset( &bi, char(0), sizeof(bi) );
| ^
...
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/ripemd160.cpp: In function ‘void fc::from_variant(const fc::variant&, fc::ripemd160&, uint32_t)’:
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/ripemd160.cpp:109:40: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::ripemd160’; use assignment or value-initialization instead [-Wclass-memaccess]
109 | memset( &bi, char(0), sizeof(bi) );
| ^
...
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/hash160.cpp: In function ‘void fc::from_variant(const fc::variant&, fc::hash160&, uint32_t)’:
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/hash160.cpp:138:37: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::hash160’; use assignment or value-initialization instead [-Wclass-memaccess]
138 | memset( &bi, char(0), sizeof(bi) );
| ^
...
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/sha256.cpp:109:40: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::sha256’; use assignment or value-initialization instead [-Wclass-memaccess]
109 | memset( &bi, char(0), sizeof(bi) );
| ^
...
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/sha224.cpp:89:40: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::sha224’; use assignment or value-initialization instead [-Wclass-memaccess]
89 | memset( &bi, char(0), sizeof(bi) );
| ^
...
/home/runner/work/bitshares-core/bitshares-core/libraries/fc/src/crypto/sha512.cpp:95:40: warning: ‘void* memset(void*, int, size_t)’ clearing an object of non-trivial type ‘class fc::sha512’; use assignment or value-initialization instead [-Wclass-memaccess]
95 | memset( &bi, char(0), sizeof(bi) );
| ^

* optional (see also: #2484)
>/home/runner/work/bitshares-core/bitshares-core/libraries/protocol/fee_schedule_calc.cpp:70:26: warning: ‘sub_asset_creation_fee’ may be used uninitialized in this function [-Wmaybe-uninitialized]
70 | optional sub_asset_creation_fee;
| ^~~~~~~~~~~~~~~~~~~~~~
...
/home/runner/work/bitshares-core/bitshares-core/tests/common/database_fixture.cpp: In member function ‘const graphene::chain::call_order_object* graphene::chain::database_fixture::borrow(const graphene::chain::account_object&, graphene::protocol::asset, graphene::protocol::asset, fc::optional)’:
/home/runner/work/bitshares-core/bitshares-core/tests/common/database_fixture.cpp:1237:32: warning: ‘*((void*)& update +64)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1237 | call_order_update_operation update = {};
| ^~~~~~
/home/runner/work/bitshares-core/bitshares-core/tests/common/database_fixture.cpp: In member function ‘void graphene::chain::database_fixture::cover(const graphene::chain::account_object&, graphene::protocol::asset, graphene::protocol::asset, fc::optional)’:
/home/runner/work/bitshares-core/bitshares-core/tests/common/database_fixture.cpp:1262:32: warning: ‘*((void*)& update +64)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
1262 | call_order_update_operation update = {};
| ^~~~~~
...
/home/runner/work/bitshares-core/bitshares-core/libraries/wallet/wallet_transfer.cpp: In member function ‘graphene::protocol::signed_transaction graphene::wallet::detail::wallet_api_impl::borrow_asset_ext(std::string, std::string, std::string, std::string, graphene::protocol::call_order_update_operation::extensions_type, bool)’:
/home/runner/work/bitshares-core/bitshares-core/libraries/wallet/wallet_transfer.cpp:239:35: warning: ‘*((void*)& op +64)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
239 | call_order_update_operation op;
| ^~
...
In file included from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/variant_object.hpp:2:0,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/log/log_message.hpp:8,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/log/logger.hpp:5,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/exception/exception.hpp:6,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/safe.hpp:2,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/io/raw_fwd.hpp:7,
from /home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/io/enum_type.hpp:3,
from /home/ubuntu/bitshares-core-dev/libraries/protocol/include/graphene/protocol/types.hpp:42,
from /home/ubuntu/bitshares-core-dev/libraries/chain/include/graphene/chain/types.hpp:26,
from /home/ubuntu/bitshares-core-dev/libraries/chain/include/graphene/chain/account_object.hpp:26,
from /home/ubuntu/bitshares-core-dev/libraries/chain/market_evaluator.cpp:24:
/home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/variant.hpp: In member function ‘graphene::db::object_id_type graphene::chain::call_order_update_evaluator::do_apply(const graphene::protocol::call_order_update_operation&)’:
/home/ubuntu/bitshares-core-dev/libraries/fc/include/fc/variant.hpp:634:17: warning: ‘old_debt.fc::safe::value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
to_variant( static_cast(s.value), v, max_depth );
^
/home/ubuntu/bitshares-core-dev/libraries/chain/market_evaluator.cpp:316:25: note: ‘old_debt.fc::safe::value’ was declared here
optional old_debt;
^

* boost unit_test
>In file included from /usr/include/boost/config/header_deprecated.hpp:18,
from /usr/include/boost/test/auto_unit_test.hpp:16,
from /home/runner/work/bitshares-core/bitshares-core/tests/performance/genesis_allocation.cpp:31:
/usr/include/boost/test/auto_unit_test.hpp:17:1: note: #pragma message: This header is deprecated. Use This header is deprecated. Please use instead. instead.
17 | BOOST_HEADER_DEPRECATED( "This header is deprecated. Please use instead." )
| ^~~~~~~~~~~~~~~~~~~~~~~

Start by reproducing the Ubuntu 20.04 build and reviewing the warnings in libraries/fc/src/crypto, libraries/protocol/fee_schedule_calc.cpp, tests/common/database_fixture.cpp, libraries/wallet/wallet_transfer.cpp, libraries/chain/market_evaluator.cpp, and tests/performance/genesis_allocation.cpp. Check the vendor/websocketpp CMakeLists.txt warning as well. Done means the listed compiler, CMake, and Boost warnings are addressed and the build remains successful.

Written by the indexing model from the issue text.