Rename cleanup_delay to cleanup_interval for connection pool

Document the change.  Make the old "cleanup_delay" still work
in the config files.

Change the expiry algorithm to be exponential decay.
The first connection is cleaned up at

	last_spawned + cleanup_interval

The next one at

	last_spawned + cleanup_interval + cleanup_interval >> 1

etc.  This should minimize connection bouncing
Operator precedence