core: improve performance through negative command caching

core: improve performance through negative command caching

Instead of incrementing the proc epoch on every command removal
and some command creation, cache previous deleted commands
(empty structure only). Periodically increment the proc epoch
and invalide all cached commands.

This is especially a win when creating short lived commands.
e.g.

	proc a {} {
		local proc b {} {
			# do something
		}
		# now b is removed
	}

Signed-off-by: Steve Bennett <steveb@workware.net.au>