LU-17379 mgc: try MGS nodes faster

Re-organize import_select_connection to try all NIDs
faster at least at first round.

- check NID LNET discovery status and skip those not
  discovered yet on first round, at next round just
  select the least recently used one
- reset AT timeout to minimal values at first round
- track per-connection total attempts to connect,
  how many were replied, discovery status and output
  this in import stats

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ib4d043e82bf156cc3e7c9ddeff0055790edcc9ee
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54022
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>