Adds QueryHandler interface

patch by beobal & slebresne; reviewed by beobal & slebresne for CASSANDRA-6659
Allow manually specifying start/end tokens in CFIF
patch by Paulo Gomes; reviewed by Piotr Kołaczkowski for CASSANDRA-6436
Restrict Windows to parallel repairs
patch by Josh McKenzie; reviewed by jbellis for CASSANDRA-6907