Modify lock to allow simultaneous transfers to different hosts.

[william-stearns]

Instead of a single lock limiting us to one transfer at a time, change the lock so each destination host has its own lock. This allows multiple transfers to be kicked off at the same time as long as they're to different machines. (Example use: 2 ACH servers at 2 different data centers)

[ethack]

I didn't test, but the code looks good to me. I can't think of any issues other than resource contention this behavior change could cause. If that turns out to be a problem this could be put behind a --parallel flag or similar.

[william-stearns]

As a brief explanation:

• The "flock" was originally placed there to give a lock to the transfer so that we couldn't have 2 copies of it running at any one time. (If we have 2, we'd send each file to the remote server twice, slowing down the transfer even more.)
• The new lock statement allows one lock per remote ssh host. In a case where a sensor will be sending logs to more than one AC-Hunter or RITA system (such as when one has a primary and a backup/disaster recovery AC-Hunter system), it's possible to have transfers running to both systems simultaneously. Both can be kicked off from cron at the same time.
• This change directly benefits one of our larger customers who needs the transfers to happen simultaneously.

[Zalgo2462]

This has been tested in a deployment and found to be effective. The change is simple, and it shouldn't affect most users. LGTM