LU-1789 protocol: add support for lightweight connection

LU-1789 protocol: add support for lightweight connection

A lightweight connection has the following uncommon properties:
- no entry is added to last_rcvd file, so no recovery is possible
- a lightweight connection can be set up while the target is in
  recovery, therefore it should be used use caution
- locks can still be acquired through this connection, although they
  won't be replayed.

We might also consider disabling pings in the future.

Signed-off-by: Johann Lombardi <johann@whamcloud.com>
Change-Id: I2b2d1a28d0fd4ca278806cab6103f96fbd88a07d
Reviewed-on: http://review.whamcloud.com/3925
Reviewed-by: Niu Yawei <niu@whamcloud.com>
Tested-by: Hudson
Reviewed-by: wangdi <di.wang@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>