Remove unused testoverlay module
Improve BuildInfo interface:
* Remove unnecessary beast::String dependency
* Explicitly cast to result type while packing a version
* Add unit tests for version formatting
Change output range names of ledger_cleaner

The input parameters are called "min_ledger" and "max_ledger", they are also called "minRange" and "maxRange" in the code BUT "ledger_min" and ledger_max" if printed. This is inconsistent and should be changed, as it might lead to confusion on how to call this module via RPC.
Improve transaction security
* Check signatures of every transaction on every validator
* Remove obsolete code
* Check transaction status in submit/sign RPC handler
Fix broken assert in built/validated ledger mismatch handler
Allow beast::lexicalCast to parse 'true' & 'false' into a bool
Miscellaneous cleanups:
* Limit HashPrefix construction and disallow assignment

* Give KnownFormats deleted copy members so that derived
  classes will give the right answers if queried with the
  std::is_copy_constructible/assignable traits.

* Replace SharedSingleton with a local static in
  LedgerFormats::getInstance() to be consistent with
  similar code in other places.  This also allows the
  LedgerFormats default constructor to be marked private
  so that the compiler enforces the design that
  LedgerFormats is a singleton type.

* Change return types of LedgerFormats::getInstance() and
  TxFormats::getInstance() from pointer to non-const to
  reference to const so as follow more established design
  guidelines for singletons.  This prevents pointers being
  mistaken for heap-allocated objects, and the const
  ensures the singleton isn't mutable.

* Change RippleAddress to inherit privately from
  CBase58Data instead of publicly.  This lets the compiler
  enforce that there are no unintended conversions from
  RippleAddress to CBase58Data.  This change allows us
  to remove a comment warning about unwanted conversions.
Fix compiler warnings under gcc.
New DatabaseReader reads ledger numbers from database.
Clean and harden Transaction.

* Replace boolean parameter with enumerated type.
* Get rid of std::ref.
* 80-column cleanups.
* Replace an std::bin with a lambda.
Remove the use of beast::String from rippled (RIPD-443)
Add counters to track nodestore read and write activities.
Make travis build and use debug variants for tests