workaround for #3482

Until all client requests are non-blocking we have to bump-up no. of threads in a client engine
thread-pool. When all threads are occupied by blocked threads then no other request can be processed.

This eventually lead to this sequence of events:
 1. a heartbeat time-out
 2. connection is closed
 3. TargetDisconnectedException is thrown

Credits for identifying the root cause go to Mehmet!

(cherry picked from commit ebdde20e91e0c92d459490495a241c9985dbc128)