This is a refactor aimed at cleaning up and simplifying the existing
job queue.
As of now, all jobs are cancelled at the same time and in the same
way, so this commit removes the per-job cancellation token. If the
need for such support is demonstrated, support can be re-added.
* Revise documentation for ClosureCounter and Workers.
* Simplify code, removing unnecessary function arguments and
deduplicating expressions
* Restructure job handlers to no longer need to pass a job's
handle to the job.
* Improve documentation
* Make the ShardArchiveHandler rather than the DatabaseShardImp perform
LastLedgerHash verification for downloaded shards
* Remove ShardArchiveHandler's singleton implementation and make it an
Application member
* Have the Application invoke ShardArchiveHandler initialization
instead of clients
* Add RecoveryHandler as a ShardArchiveHandler derived class
* Improve commenting
* Make ShardArchiveHandler a singleton.
* Add state database for ShardArchiveHandler.
* Use temporary database for SSLHTTPDownloader downloads.
* Make ShardArchiveHandler a Stoppable class.
* Automatically resume interrupted downloads at server start.
* Reduce lock scope on all public functions
* Use TaskQueue to process shard finalization in separate thread
* Store shard last ledger hash and other info in backend
* Use temp SQLite DB versus control file when acquiring
* Remove boost serialization from cmake files
