guesstimate raw e2e latency and subtract from target chunk time

[morgo]

The main goal of setting a target chunk time is to limit the maximum time row locks are held during chunk copies. Since chunk copies are performed in auto_commit the e2e latency from spirit (the client) never factors.

Because spirit is reasonably network efficient (it performs most of the work on the server - the main exception being reading binary logs) it might be desirable to have a central spirit server which performs migrations for a remote region. i.e. have spirit in us-west-2 run migrations for us-west-2 and us-east-1.

A way to make it require less configuration is to guess the e2e latency by periodically issuing a "select 1" query. This time can then be subtracted from the chunk copy time when coming up with new target times.