Connection pool: per-operation metadata_mode

`connection_pool` doesn't allow setting metadata mode, so there's no way to retrieve metadata strings from a pooled connection (the static interface still works, since it bypasses metadata mode).

Having a per-connection metadata mode is error prone, and a leftover from the time when we didn't have the universal `execute` and `start_execution` functions. Consider a per-operation metadata mode, deprecating `meta_mode` and `set_meta_mode` from `connection`, and removing the latter functions from `any_connection`.