evaluations for validated ledger age.

* Document delete_batch, back_off_milliseconds, age_threshold_seconds.
* Convert those time values to chrono types.
* Fix bug that ignored age_threshold_seconds.
* Add a "recovery buffer" to the config that gives the node a chance to
  recover before aborting online delete.
* Add begin/end log messages around the SQL queries.
* Add a new configuration section: [sqlite] to allow tuning the sqlite
  database operations. Ignored on full/large history servers.
* Update documentation of [node_db] and [sqlite] in the
  rippled-example.cfg file.
* Resolves XRPLF#3321

(Include these items in the main commit when squashing.)
* Update rippled URLs in documentation.
* Full history and fetch depth use max() value instead of an arbitrary
  value of 1 billion.

* changed the sqlite ledger_history cutoff to log a warning instead of
  ignoring sqlite settings. updated documentation.
* integrated the commonpragma back in to the setup struct and restored
  the interface of the databasecon ctors.

* involved moving some test logging subclasses around to be more easily
  accessible, and some minor refactoring of Env.

* Make `safety_level` and other tuning options mutually exclusive
* Clean up VACUUM functionality and tuning to use settings.
* Fix bugs in the config setup where it still ignored tuning settings if
  full history. Add tests to catch that mistake.
* Fix some bugs in the unit tests, and make them more robustly check
  error messages.

* Change some log message levels.
* clearSql no longer returns a bool that every caller ignores.
* Restructure a few unnecessary validity checks

* Rename retry_buffer_seconds to retry_wait_seconds.
* Improve documentation and examples in example.cfg file.
* Simplify the DatabaseCon::Setup pragma handling.
* Other misc cleanups

* Improved some of the documentation and log messages
* Removed the unnecessary `DatabaseCon::Base` ctor helper class