Named counter server

Requests issued using the CALL interface to the named counter server are not made to wait during rebuild, but instead the server returns a new environment error return code with value 311, under the NC_ENVIRONMENT_ERROR category.

An EXEC interface request during rebuild waits using a timer wait and retry loop which can be interrupted by an operator purge command but is not eligible for DTIMOUT, since the wait is definitely known not to be caused by any form of deadlock.