Log Message:
gdbm: update to 1.20.

Version 1.20, 2021-06-17

* New bucket cache

The bucket cache support has been rewritten from scratch.  The new
bucket cache code provides for significant speed up of search
operations.

* Change mmap prereading strategy

Pre-reading of the memory mapper regions, introduced in version 1.19
can be advantageous only when doing intensive look-ups on a read-only
database.  It degrades performance otherwise, especially if doing
multiple inserts.  Therefore, this version introduces a new flag
to gdbm_open: GDBM_PREREAD.  When given, it enables pre-reading of
memory mapped regions.

See https://github.com/Perl/perl5/issues/18884 for details.