Check if worker is alive for clean_mailbox

When a connection:close header is sent from the server, we handle it
by calling ibrowse:stop on the worker and release it from the worker
pool. But our clean_mailbox tries to clean the mailbox of this worker
when it's already dead, leading to a timeout that crashes the
changes_reader process and subsequently the replicator process. So
we check to ensure that the Worker is still alive before we call
ibrowse:stream_next.

BugzId:69053