mroonga_enable_reference_count: add a new variable
mroonga_enable_reference_count: add a new variable

It's for changing Groonga's reference count mode via MySQL's variable.

You can set the variable by "SET GLOBAL mroonga_enable_reference_count
= ON" but it's just ignored. Because Mroonga always opens at least one
Groonga database internally.

You need to add "loose-mroonga-enable-reference-count = ON" to your
my.cnf to enable Groonga's reference count mode via MySQL's variable.

TODO: Document

mroonga/mroonga,mroonga/mroonga,mroonga/mroonga,mroonga/mroonga