Added native stubs and made changes for Beta 3 (API version 23)

* Removed fdb_c.dll from the solution (not needed anymore since LoadLibrary finds the one installed by the SDK)
* Add stubs for new methods
* fdb_future_is_error => fdb_future_get_error
* FdbTransaction.WatchAsync(...)
* Added FdbMutationType and FdbConflictRangeType enums