that innodb_commit_concurrency>0 remains constant at run time. (Bug #42101)

srv_commit_concurrency: Make this a static variable in ha_innodb.cc.

innobase_commit_concurrency_validate(): Check that innodb_commit_concurrency
is not changed from or to 0 at run time.  This is needed, because
innobase_commit() assumes that innodb_commit_concurrency>0 remains constant.
Without this limitation, the checks for innodb_commit_concurrency>0
in innobase_commit() should be removed and that function would have to
acquire and release commit_cond_m at least twice per invocation.
Normally, innodb_commit_concurrency=0, and introducing the mutex operations
would mean significant overhead.

innodb_bug42101.test, innodb_bug42101-nonzero.test: Test cases.

rb://123 approved by Heikki Tuuri