Macro the db.ex function wrappers

Disclaimer: good first issue if you're not knowleadgable about the protocol, but macros are not trivial elixir. If you have worked with macros in any language but are new to ethereum consensus this might be a good first issue.

Most wrappers in the db.ex have the following pattern:

def some_function(some_args) do
  ref = GenServer.call(@registered_name, :get_ref)
  Exleveldb.some_function(some_args)
end

This makes adding each function in leveldb a bit of an unnecessary effort. Ideally we should have a macro that allowed us to do something like this:

wrapper function_a(a_args)
wrapper function_b(b_args)

That automatically did the boilerplate for a single function. This ticket will be finished when both the macro is coded and also all functions in Exleveldb are wrapped.

Start by reading db.ex and the existing wrappers that call GenServer.call(@registered_name, :get_ref) before delegating to Exleveldb. Define the macro around the repeated wrapper pattern, then replace the individual wrappers so every Exleveldb function is covered; the issue is done when the macro is coded and all functions are wrapped.