LU-521 lnet: make LST support variable page size

LNet selftest can't support variable page size because it sends
number of pages over the wire.
We have to change wire format to fix this but it will have
compatibility issue, so this patch also implemented "session features"
for LST to resolve compatibility issue, only new version LST
can understand new bulk RPC format, although new LST nodes still can
communicate with old LST nodes.

The Variable page size feature can be turned on by setting
LST_FEATURES to LST_FEAT_BULK_LEN (1). This feature is off by default.

Please see the Jira ticket (LU-521) for more details.

Change-Id: I4a552a3310cf0ed0a2f5ae29eaf789469e1c245a
Signed-off-by: Liang Zhen <liang@whamcloud.com>
Signed-off-by: Doug Oucharek <doug@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1338
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>