* Add database for message bodies.

* Add database for message bodies.

* Modify handling of message-header database so that messages are
  keyed by ID as well as by number.

* Purge obsolete headers from the cache and database when the server
  info is updated.

* Change format in which headers are stored in database to improve
  performance.

* Use gdbm_fastmode for database files at all times.

* Various small performance tweaks to header-parsing code.

* Change thread organizing code to separate the switch for context
  headers from the switch allowing server probes.