logging: Fix to avoid excessive logging.

mem_get function: Log message related to mem_pool 
calloc is removed  as its been calculated in mempool
'stats'.  This messgae is consuming nearly half of 
the total log messages in DEBUG mode.

dht_hash_compute function: Changed log level from DEBUG 
to TRACE.

client_fdctx_destroy function: Changed log  level from DEBUG 
to TRACE.

Change-Id: Ic948db0419e76df4e95ebd0cabaf66eadbaada6b
BUG: 966851
Signed-off-by: Venkatesh Somyajulu <vsomyaju@redhat.com>
Reviewed-on: http://review.gluster.org/5086
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>