This makes both the open() and open_connection() methods on
Geary.DB.Database asynchronous, which allows the VersionedDatabase
open_background() and its hackery to be removed, and upgrades to be
performed asynchronously as well. It also adds a
exec_transaction_async() method to Connection, allowing an existing
object to also be used to establish an async transaction.
* src/engine/db/db-connection.vala (Connection): Add
exec_transaction_async method, update doc comments.
* src/engine/db/db-database.vala (Database): Make open and
open_connection async by executing SQLite code in a background thread,
update call sites. Move job management code out of
exec_transaction_async into a new internal add_async_job() method so it
can be used by Connection. Add unit tests.
* src/engine/db/db-transaction-async-job.vala (TransactionAsyncJob): Add
an optional internal connection method and make the job's cancellable
an internal property so a Connection instance can specify itself for
the transaction.
* src/engine/db/db-versioned-database.vala (VersionedDatabase): Remove
open_background() hack since open() is now async. Make version upgrade
hooks for derived classes async and update call sites. Use a
Nonblocking.Mutex rather than GLib mutex so upgrade exclusion works
asynchronously. Add unit tests.
* src/engine/imap-db/imap-db-database.vala (Database): Make database
upgrade methods async and execute SQL in async instructions now that
the bases classes support it. Add unit tests.
