Check for user agent string length and trim it if necessary

Summary: Fix issues raised at https://reviews.bitcoinabc.org/T55

Test Plan: create a unit test to check userAgent() return value

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

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D287