LU-12815 socklnd: add conns_per_peer parameter

LU-12815 socklnd: add conns_per_peer parameter

Introduce conns_per_peer ksocklnd module parameter.
In typed mode, this parameter shall control
the number of BULK_IN and BULK_OUT tcp connections,
while the number of CONTROL connections shall stay
at 1. In untyped mode, this parameter shall control
the number of untyped connections.
The default conns_per_peer is 1. Max is 127.
Performance scaling on 100GbE:

 conns_per_peer     speed
        1        1.7GiB/s
        2        3.3GiB/s
        4        6.4GiB/s
        8       11.5GiB/s

Lustre-change: https://review.whamcloud.com/41056
Lustre-commit: 71b2476e4ddb95aa42f4a0ea3f23b1826017bfa5

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I1f4ef22141882224e14e18c2526554dcfa69c871
Reviewed-on: https://review.whamcloud.com/41411
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>