database/sql: optimize connection request pool

database/sql: optimize connection request pool

This replaces a map used as a set with a slice.

We were using a surprising amount of CPU in this code, making mapiters
to pull out a random element of the map. Instead, just rand.IntN to pick
a random element of the slice.

It also adds a benchmark:

                     │    before    │                after                │
                     │    sec/op    │   sec/op     vs base                │
    ConnRequestSet-8   1818.0n ± 0%   452.4n ± 0%  -75.12% (p=0.000 n=10)

(whether random is a good policy is a bigger question, but this
 optimizes the current policy without changing behavior)

Updates #66361

Change-Id: I3d456a819cc720c2d18e1befffd2657e5f50f1e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/572119
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>