Revert "free the worker if it's still marked as busy."

This reverts commit 95464a684671fac9f654eb3977ba1d1a3a03e979.

Conflicts:
	apps/couch_replicator/src/couch_replicator_httpc_pool.erl
miscellaneous replicator fixes

backport from
https://github.com/rcouch/couchdb-couch-replicator/commit/f76353874d5c0f86f99f17ea87c2e010278c1eae

- monitor clients and make sure to remove the worker from the pool when
  needed
- prevent unexpected retry
- handle the case where the doc is missing