Document the behavior change in IMPALA-7800

The current implementation of the FE thrift server waits
indefinitely to open the new session, if the maximum number of
FE service threads specified by --fe_service_threads has been
allocated.

This patch introduces a startup flag to control how the server
should treat new connection requests if we have run out of the
configured number of server threads.

If --accepted_client_cnxn_timeout > 0, new connection requests are
rejected by the server if we can't get a server thread within
the specified timeout.

We set the default timeout to be 5 minutes. The old behavior
can be restored by setting --accepted_client_cnxn_timeout=0,
i.e., no timeout. The timeout applies only to client facing thrift
servers, i.e., HS2 and Beeswax servers.