* Distinguish between recent and historical shards
* Allow multiple storage paths for historical shards
* Add documentation for this feature
* Add unit tests
There was a race condition in `on_accept` where the object's destructor
could run while `on_accept` was called.
This patch ensures that if `on_accept` is called then the object remains
valid for the duration of the call.
* 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.