Use ThreadPoolExecutor for max_concurrent_live_migrations

This changes the max_concurrent_live_migrations handling
to use a ThreadPoolExecutor so that we can control a bounded
pool of Futures in order to cancel queued live migrations
later in this series.

There is a slight functional difference in the unlimited
case since starting in python 3.5, ThreadPoolExecutor will
default to ncpu * 5 concurrently running threads. However,
max_concurrent_live_migrations defaults to 1 and assuming
compute hosts run with 32 physical CPUs on average, you'd
be looking at a maximum of 160 concurrently running live
migrations, which is probably way above what anyone would
consider sane.

Co-Authored-By: Matt Riedemann <mriedem.os@gmail.com>

Part of blueprint abort-live-migration-in-queued-status

Change-Id: Ia9ea1e164fb3b4a386405538eed58d94ad115172

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