Add option to prevent automatic connection closure

[ArcanoxDragon]

My application shares a MySQL connection with express-mysql-session and handles closing the connection by itself. I need to have express-mysql-session clear its expiry interval so the application doesn't hang, but I don't want my SQL connection to be closed when I call store.close(). This new option defaults to current behavior so this should not be a breaking change.

[chill117]

I assume you aren't using clearExpirationInterval() because you are concerned with it not being explicitly a "public" method? We could add another method that would be intended for public use; perhaps something like stopClearingExpiredSessions.

[ArcanoxDragon]

I just realized this PR is probably awaiting my reply...

I suppose I can use clearExpirationInterval(), but as you said, I wasn't sure if it was necessarily a "public" method that was safe to rely on API-wise.

[chill117]

Yes, feel free to use clearExpirationInterval. It won't be changed.