Do not check if a write socket is ready for read. #4735

Previously, after any socket has been serviced, DC would check all
subsequent sockets if they are still ready for reading before a handler
is called (the theory is that a prior handler may have drained the socket
and the current handler is blocking).

This did not play well with write sockets; they were never ready for read,
meaning that write sockets were only serviced during otherwise-idle DC driver
loops (these can be very rare on a busy schedd!).

As a solution, we only have the "recheck" of sockets registered for read.
Allow schedd to fork when servicing the V3 query protocol. #4735

The V3 query protocol could be slow if the schedd infrequently serviced
its sockets.  With this commit, the schedd will fork off response
handlers until it hits the SCHEDD_QUERY_WORKERS limit.  Subsequent
queries will be handled by the parent process.

Sites very concerned about memory usage of the schedd can set
SCHEDD_QUERY_WORKERS=0 and still be able to service requests - but with
a performance hit under load.