Bug#21413
"Engine table handler used by multiple threads in REPLACE DELAYED"
When executing a REPLACE DELAYED statement, the storage engine
::extra() method was invoked by a different thread than the thread
which has acquired the handler instance.

This did not cause problems within the current server and with
the current storage engines.
But it has the potential to confuse future storage engines.

Added code to avoid surplus calls to extra() method in case of DELAYED
which avoids calling storage engine from a different thread than
expected.

No test case.
This change does not change behavior in conjunction with current
storage engines. So it cannot be tested by the regression test suite.