Decouple schema fetch queries from server-side timeouts

[dkropachev]

Rust driver part should be completed under scylladb/scylla-rust-driver#1052

Here we need only expose configuration option.

[dkropachev]

scylla-rust-driver got it implemented scylladb/scylla-rust-driver#1073
cpp-rust-driver will get it automatically, but we need API to configure default timeout