mth plugin: make it immune from being forgotten
mth plugin: make it immune from being forgotten

allocate temporary structures as ruby objects, so that it will
be collected even if it's forgotten inside of fiber.

Change-Id: Ia8576f1790f208826d3f8fa06f2a2c19f5bd2074
Reviewed-on: http://review.couchbase.org/23529
Reviewed-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>
Tested-by: Sergey Avseyev <87f6d5e4fd3644c3c20800cde7fd3ad1569370b3@gmail.com>

dewolfe/couchbase-ruby-client,dewolfe/couchbase-ruby-client