Refactor protocol pauser

https://github.com/DistributedCollective/Sovryn-smart-contracts/pull/311
as discussed primarily with @smbsp
e.g. in _setTarget(this.swapExternal.selector, target);
this.swapExternal.selector
is replaced for
this.swapExternal.selector^this.xor.selector (which should be a constant - )
in all modules
that will lead to calling non-existing functions from the protocol meaning that all the calls will be transferred to the fallback function
so, add fallback functions to all modules which will serve as a hook prior to calling any module function

in the fallback function:
- check that the protocol is not on pause and virtually any preliminary processing
- xor func sig again to restore true func sig
- check that the true sig exists
- call the function with the tru func sig

verify

• possible gas limit with the fallback function
• check each function call gas overhead