ctdb-io: Do not use sys_write to write to client sockets

ctdb-io: Do not use sys_write to write to client sockets

When sending messages to clients, ctdb checks for EAGAIN error code and
schedules next write in the subsequent event loop.  Using sys_write in
these places causes ctdb to loop hard till a client is able to read from
the socket.  With real time scheduling, ctdb daemon spins consuming 100%
of CPU trying to write to the client sockets.  This can be quite harmful
when running under VMs or machines with single CPU.

This regression was introduced when all read/write calls were replaced to
use sys_read/sys_write wrappers (c1558adeaa980fb4bd6177d36250ec8262e9b9fe).

The existing code backs off in case of EAGAIN failures and waits for an
event loop to process the write again.  This should give ctdb clients
a chance to get scheduled and to process the ctdb socket.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Martin Schwenke <martin@meltin.net>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Tue Feb 24 12:29:30 CET 2015 on sn-devel-104