upgrade rust-postgres and support single-packet queries?

[tmm1]

the tokio-postgres dep is currently pinned to a 0.7.7 fork: https://github.com/prisma/rust-postgres/commits/pgbouncer-mode/

the latest version is 0.7.11: https://github.com/sfackler/rust-postgres/blob/master/tokio-postgres/CHANGELOG.md

the newer versions include a query_typed which reduces the number of network packet roundtrips for a fresh query from 3 to 1: sfackler/rust-postgres#1147

cc @aqrln
cc #5017

[tmm1]

cc @janpio sfackler/rust-postgres#1067 (comment)

We at @prisma would also be very interested in this.
Right now, we do a weird dance of wrapping all queries in transactions, and each time deallocating the named prepared statements when talking to a PgBouncer instance in transaction mode - which adds up to 3 additional roundtrips for 1 query and has an obvious very bad effect on our performance.

[aqrln]

Oh, I just realized this also solves the problem of the traceparent comment invalidating prepared statement cache with tracing enabled, wonderful.