quantizer: support MySQL comments

[palazzem]

Overview

Updates a contributor PR: #320

It adds support for MySQL comments (inline especially) that are not properly removed from the Agent quantizer:

--- FAIL: TestSQLQuantizer (0.00s)
        Location:       sql_test.go:307
	Error:		Not equal: "select * from users where id = ?" (expected)
			        != "select * from users where id = ? # This comment continues to the end of line" (actual)

This patch solves the problem.

[LotharSee]

Great fix, I thought we were already doing it :/
Test coverage is good. LGTM!