Add a new global server system variable mroonga_lock_timeout

You can change Groonga's lock timeout dynamically.

TODO: Document it. See Groonga's documentation of
grn_set_lock_timeout() and grn_get_lock_timeout() to write
mroonga_lock_timeout's documentation.

NOTE: MySQL 5.5 has a bug that MySQL always reports an invalid value
error for negative value for integer system variable. So we can't
disable timeout by -1. "Disabling timeout" means that Groonga retries
acquiring a lock until Groonga acquiring a lock.