ActiveStreamCheckpointProcessorTask::run keeps readLock on StreamContainer

https://github.com/couchbase/kv_engine/blob/3071fdcc090aa6974c87c14d0340d9753eab0322/engines/ep/src/dcp/active_stream_checkpoint_processor_task.cc#L49-L53

We've seen DCP_STREAM_REQUEST execute slowly and a suspect was the StreamContainer data structure which is used twice per request

1) A look-up (does a stream already exist) - read-lock only
2) An insert (new stream placed in map) - write-lock

In a same set of logs we see a slow-runtime for ActiveStreamCheckpointProcessorTask just before a bunch of DCP_STREAM_REQUESTS are released (also logging as slow).

The logs in question showed a CBAS workload reconnecting, which is a heavy user of the stream-id feature which means a stream-container can hold many objects.

I suspect that alongside maybe some large checkpoints the read-lock is held for quite sometime - although in the logs we see a 17 second runtime for the stream processor which is worrying, that would have to be a huge checkpoint.

However it's clear that the ActiveStreamCheckpointProcessorTask should not keep a read-lock whilst processing all of the streams, it should copy the ActiveStream (shared_ptr) and drop the lock then process.

Suspect all collection releases affected, will check.