It is done.

Initial implementation of the new database subsystem

These pieces represent the foundation for ticket #5034

Expanded transactions, added VersionedDatabase

Further expansions of the async code.

Moved async pool logic into Database, where it realistically
belongs.

Further improvements.  Introduced geary-db-test.

Added SQL create and update files for Geary.Db

version-001 to version-003 are exact copies of the SQLHeavy scripts
to ensure no slight changes when migrating.  version-004 upgrades
the database to remove the ImapFolderPropertiesTable and
ImapMessagePropertiesTable, now that the database code is pure
IMAP.

When we support other messaging systems (such as POP3), those
subsystems will need to code their own database layers OR rely on
the IMAP schema and simply ignore the IMAP-specific fields.

ImapDB.Account fleshed out

ImapDB.Folder is commented out, however.  Need to port next.

ImapDB.Folder fleshed out

MessageTable, MessageLocationTable, and Atta...