Destroy lost connection objects acquired from mysql.connect()

[Totktonada]

When a connection is acquired from a pool, lost and collected by Lua GC, its underlying connection will be closed from a GC callback. A connection that is created directly w/o a pool using mysql.connect() has no such GC callback, so its underlying connection will not be properly closed if a user does not call <connection object>:close() manually.

A fiber may store a connection in its local variable. If this fiber does not handle abnormal exit properly and, say, does not close the connection when is hit by fiber.cancel(), the underlying connection will leak.

We should test than the underlying connection is closed in the case.

[romanhabibov]

I've checked. Connection doesn't leak actually. lua_mysql_gc(struct lua_State *L) is called with collectgarbadge(). The ticket isn't relevant.

mysql/mysql/driver.c

Line 529 in 6f785f7

lua_mysql_gc(struct lua_State *L)

[Totktonada]

How about a test that checks that a file descriptor actually closed?