Implement "EBx.y" indicator on subversion (user agent)

Summary:
Add excessiveBlockSize indicator in the subversion string in MB.
The value would be truncated to the first decimal digit.

Following two example of what the subver will be in case of
different EB settings:

1. excessiveBlockSize set to 2000000 --> /Bitcoin ABC:0.14.1(EB2)/

2. excessiveBlockSize set to 1560000 --> /Bitcoin ABC:0.14.1(EB1.5)/

Add also functional tests for subver EB signalling to abc-{cmdline,rpc}.py

Test Plan: run abc-{cmdline,rpc}.py

Reviewers: #bitcoin_abc, freetrader, deadalnix, kyuupichan, awemany

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D222