Log the duration a connection is paused by MaxSendQueueSize

Follow-up from MB-60706.

Initial draft at https://review.couchbase.org/c/kv_engine/+/205383.

As discussed on Slack:

In place of a counter calculate the duration at which the connection is blocked due to a full send queue. For example, introduce the following two stats:

std::chrono::nanoseconds blocked_on_full_send_queue_duration;

std::optional<std::chrono::steady_clock::time_point> blocked_on_full_send_queue; 

And update them on entry and exit of Connection::executeCommandsCallback  and Connection::processNotifiedCookie (and there might be others).