Prevent reclaim in send_traverse_thread()

As is the case with traverse_prefetch_thread(), the deep stacks caused
by traversal require disabling reclaim in the send traverse thread.

Also, do the same for receive_writer_thread() in which similar problems
have been observed.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4912
Closes #4998