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.
The existing platform detection code was derived from the old Beast
library, which was, itself, derived from JUCE.
This commit removes that code and replaces it with the Boost.Predef
library which defines a consistent set of compiler, architecture,
operating system, library, and other version numbers.
For more on Boost.Predef, please see the Boost documentation. The
documentation for the current version as of this writing is at:
https://www.boost.org/doc/libs/1_71_0/doc/html/predef.html
