Update devel/mtbl to version 1.2.1:

• Prevent fileset reloading when the fileset has iterators open.
• Add and document facility to disable reloading of filesets.
• Defer initial load of fileset until the first operation on a fileset source.
• Handle 32bit size_t overflows, failing with an assertion upon opening an mtbl_reader with an oversized data block.
• Fix for systems with 32-bit size_t.
• Add several unit tests for various libmtbl functions.
• Correct a library versioning error.

Sponsored by: Farsight Security, Inc.