* Bucket cache switched from balanced tree to hash table

Change suggested by Terence Kelly.

* Speed up flushing the changed buckets on disk

* New option codes for gdbm_setopt

** GDBM_GETDBFORMAT

Return the database format.

** GDBM_GETDIRDEPTH

Return the directory depth, i.e. the number of initial (most significant)
bits in hash value that are interpreted as index to the directory.

** GDBM_GETBUCKETSIZE

Return maximum number of keys per bucket.

** GDBM_GETCACHEAUTO

Return the status of the automatic cache adjustment.

** GDBM_SETCACHEAUTO

Enable or disable automatic cache adjustment.