devel/mtbl: Upgrade to version 1.7.1

1.7.1:

• Incremented version for new tarball.

1.7.0:

• Add public mtbl_threadpool API, enabling multithreaded functionality for mtbl_writer and mtbl_sorter objects; see mtbl_threadpool manpage for more info.
• Add mtbl_writer_options_set_threadpool() to enable multithreaded data block compression.
• Add mtbl_sorter_options_set_threadpool() to enable multithreaded sorting.
• Add -t option for setting the preferred number of threads to use in an mtbl_threadpool object used for writing in mtbl_merge.

Bump PORTREVISION of dependent ports.

Sponsored by: DomainTools LLC