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 AttachementTable are now
handled inside ImapDB.Folder.

chmod -x imap-db-database.vala

OutboxEmailIdentifier/Properties -> SmtpOutboxEmailIdentifier/Properties

Moved SmtpOutboxFolderRoot into its own source file

SmtpOutboxFolder ported to new database code

Move Engine implementations to ImapDB.

Integration and cleanup of new database code with main source

This commit performs the final integration steps to move Geary
completely over to the new database model.  This also cleans out
the old SQLHeavy-based code and fixes a handful of small bugs that
were detected during basic test runs.

Moved Outbox to ImapDB

As the Outbox is tied to the database that ImapDB runs, move the
Outbox code into that folder.

Outbox fixes and better parameter checking

Bumped Database thread pool count and made them exclusive

My reasoning is that there may be a need for a lot of threads at
once (when a big batch of commands comes in, especially at
startup).  If performance looks ok, we might consider relaxing
this later.