handlerton refactoring

Problems:

• it's the only way to participate in transactions
  • there are few fake storage engines that aren't storage engines but pretend to be (with the HTON_NOT_USER_SELECTABLE | HTON_HIDDEN flags) solely to be able to participate in transactions. Binlog, online alter log and vector indexing, for example.
• initializing handlerton dynamically in _init is a bit annoying, always passing itself as a first argument is too. This is a consequence of a decision to use C struct.

It could be changed to C++ class handlerton. That inherits from a parent C++ class "transaction participant". Binlog, online alter and vector indexing will only need to use the latter.