XA - Explicit startTransaction() call

Resource managers (now currently only storage engines) must manually track whether or not they are "inside" a transaction (either a statement or a normal transaction). This is typically done by incrementing and decrementing a counter that the storage engine keeps when StorageEngine::external_lock(F_RDLOCK|F_WRLCK) and StorageEngine::external_lock(F_UNLCK) are called.

This API is needlessly complex and should be replaced with explicit calls to the resource manager telling the resource manager when a transaction starts and when it ends:

XaResourceManager::startTransaction();
XaResourceManager::endTransaction();