Clean up CKey and RippleAddress (RIPD-672)

* Remove CKey dependency on RippleAddress
* Create RAII ec_key wrapper that hides EC_KEY and other OpenSSL details
* Move CKey member logic into free functions
* Delete CKey class
* Rename units that are no longer CKey-related
* Delete code that was unused

This tree is identical to that at commit 82687254, the last in <https://github.com/ripple/rippled/pull/720>.

@vinniefalco