Add UniqueId.maxPossibleId() to determine the max ID

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>
Add UniqueId.getUsedUIDs() to fetch all of the max UIDs in one call
asynchronously as per isavin's work on #153

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>
Modify TSDB.collectStats() to return the used and available UIDs as per isavin's
work on #153

Signed-off-by: Chris Larsen <clarsen@euphoriaaudio.com>