MB-10908 upr client: Remove user docs from crash log process state

When upr client's gen server crashes, the crash logger will try to dump
entire gen server state. The stream queues contain user documents and it
is part of gen server state. Dumping lot of user documents may cause high
memory usage for crash logger. Added a format callback to prevent all docs
from stream queues being dumped into the crash logs.

Change-Id: Iaf24552b1916d9c66d1cf1dbf4bbff518aa3f0e5
Reviewed-on: http://review.couchbase.org/36289
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>