Prevent wrapping in setexcessiveblock RPC

Summary:
If the blocksize parameter is a negative integer (not passed as a
string), it wraps and sets the incorrect value.
Incidently it removes the last use of boost::lexical_cast after D4080.

Test Plan:

ninja all check
./test/functional/test_runner.py abc-rpc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5136