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`.

Start by tracing the connection_pool operation path and the existing metadata-mode APIs on connection and any_connection. Compare how execute and start_execution handle metadata mode, then determine the deprecation and removal scope described in the issue. Done means pooled operations can retrieve metadata strings without per-connection state and the affected APIs have the intended lifecycle.

Written by the indexing model from the issue text.