Merge pull request #158 from jaredj/abstract-dbm

Clarify DBM-specific functions
Drop support for per-recipient greylist dbm files

This does not drop support for including the recipient in the greylist key.

Note that this was already broken for an indeterminate (long) amount of time
Drop documentation too
Add POD section pointing to per-recip db support
Merge pull request #159 from jaredj/drop-recip-dbs

Drop support for per-recipient greylist dbm files