* Creates a version 2 of the UNL file format allowing publishers to
pre-publish the next UNL while the current one is still valid.
* Version 1 of the UNL file format is still valid and backward
compatible.
* Also causes rippled to lock down if it has no valid UNLs, similar to
being amendment blocked, except reversible.
* Resolves#3548
* Resolves#3470
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
