RCBC-166 fix crash with eventmachine

In eventmachine event handlers are separated and run seprately and in
the following order: [READ, WRITE]. So it was possible to cancel WRITE
event handler from the READ handler which cause crash when the reactor
run it in next turn.

The fix tracks current handler and deffers cancellation of WRITE
handler

Change-Id: I07b463cc3a7e368171437450d8773f67220d2f04
Reviewed-on: http://review.couchbase.org/34738
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>