Decouple schema fetch queries from server-side timeouts

[dkropachev]

If the user set server-side timeouts, or service level timeouts, this impacts schema fetches. The timeout can be too low to allow a schema fetch when the schema is very large.
Instead, qualify schema (and other metadata) fetches with USING TIMEOUT (with a high and configurable default) so schema fetches work regardless of timeout tuning.

rust-driver part of origin issue #

[dkropachev]

Dup of #1052