Perform all deletes in threads

Have all node deletions (like node launches) handled by a thread,
including ones started by the periodic cleanup.  This will make
the system more responsive when providers are unable to delete
nodes quickly, as well as when large numbers of nodes are deleted
by an operator, or when the system is restarted while many node
deletions are in progress.

Additionally, make the 'nodepool delete' command merely update the
db with the expectation that the next run of the cleanup cron will
spawn deletes.  Add a '--now' option so that an operator may still
delete nodes synchronously, for instance when the daemon is not
running.

Change-Id: I20ce2873172cb1906e7c5832ed2100e23f86e74e