Pause contracts

[smbsp]

Implementation Details: https://docs.google.com/document/d/1Qe92Q_1t3Ajwvurd9_8RtV8RMGaq30hNyij9_g2QWYo/edit

Needs more testing.

closes: #210

[tjcloa]

update
moved to a separate task "Refactor protocol pauser #319"

here is an overall idea of implementation
as discussed primarily with @smbsp

1. _setTarget(this.swapExternal.selector, target);
   is replaced for
   _setTarget(this.swapExternal.selector>>1, target);
   in all modules
   that will lead to calling non-existing functions from the protocol, but this is controllable (see next)
2. add fallback functions to all modules which will serve as a hook prior to calling any module function
3. in the fallback function

• check that the protocol is not on pause and virtually any preliminary processing can be done there
• unshift func sig to get the real one: msg.sig<<1
• check that the unshifted (msg.sig<<1) sig exists
• call the func with unshifted (msg.sig<<1) signature

[korepkorep]

Several code quality issues that don't endanger contracts` security