Plumb deadline timer into mod_exp implementation

[arhag]

This allows us to remove the subjective limitation on bit size introduced with #361 since the deadline timer would be able to interrupt the computation done internally for mod_exp.

Requires code changes to be made to the math implementation of mod_exp which must wait until we replace that implementation from gmp to something with a friendlier license like BoringSSL (captured in #769).

Dependencies:

• eliminate openssl dependency for consensus bits #769

[arhag]

Replaced by AntelopeIO/leap#1255.