Reorder mutex locks - the ImageInfo ReadWriteLock must remain an "inner", transparent mutex.

It is called from a place where DatabaseAccess is locked, so DatabaseAccess must not be
locked from a place where the ImageInfo lock is held.
To reduce by one or two mutex locks, consolidate read operations at one place and
return the hash from a local variable which is in sync with the cached hash.
(note: we do not rule out a parallel operation between the DatabaseAccess and the
writing of the cached values. it's so far acceptable for a read cache.)

CCMAIL: mike@mghansen.de
BUG: 331578