Add timeout to batch put.

Batch put can now be called with with waitflush=timeout_in_seconds. The request
will complete when there are items with errors (http 400), when HBase errors
in /api/put will send http 500 errors (instead of 400), when all requests are
written to HBase (http 200) or after timeout_in_seconds (http 408).
Additionally, moved logging for all urls starting with /api/put calls debug.

NOTE: Submitted on behalf of third-party contributor:
  * Guenther Schmuelling <schmuell@pepperdata.com>