caches: extract some config reading in 'shouldwarmcache'

caches: pass the transaction to the "shouldwarncache" logic This will allow to have smarter mode about cache warming (eg: only warm them for server transaction.

caches: record 'desc' attribute on transaction This is useful to know if a transaction if server side or not.

caches: protect against missing connection If the connection cannot be established, we should not try to use it.