mod_s2s_keepalive: Invert check to work with bidi connections When mod_s2s_bidi is active there may be s2sin connections in the s2sout map, which made this not work correctly.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```