extent scan: fix crawl_map creation

extent scan: fix crawl_map creation

There are two crawl_maps in extent scan's next_transid:  one gets
initialized, the other gets used.  This works OK as long as bees is
resuming an existing scan, because the two maps are identical; however,
but it fails if bees is starting without an existing set of crawl data,
and one of the two maps is empty or partially filled.

The failure is intermittent, as the crawl map is being populated at
the same time next_transid runs.  It will eventually be completed after
several transaction cycles, at which point bees runs normally.
It does add significant delays during startup for benchmarks.

There's only one crawl_map in extent scan, it always has the same
crawlers, and extent scan's `next_transid` creates it by itself.
Ignore the map from BeesRoots/BeesCrawl.

Also throw in some missing but helpful trace statements.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>