RCBC-141 Initialize event indexes correctly

The plugin didn't trace event callbacks, which might lead to invalid
memory access during rebalance, where libcouchbase creates/removes a
lot of events because of fast-changing topology

Change-Id: If5b64eb8e23e03885f35f3c44636821f8131a507
Reviewed-on: http://review.couchbase.org/28790
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>