database/sql: cherry-pick a couple 1.23 fixes

[bradfitz]

commit e39af550f8ea57504510dc5d5fa70ba934f16fa0
Author: apocelipes <[email protected]>
Date:   Wed Mar 20 05:33:46 2024 +0000

    database/sql: fix memory leaks in Stmt.removeClosedStmtLocked
    
    Zero out elements before shrinking the slice to avoid memory leaks.
    
    Fixes #66410
    
    Change-Id: I8f64c21455761f7f7c8b6fee0b6450b98f691d91
    GitHub-Last-Rev: b15586e801199b9674f5bfcb12a848a55f15a80b
    GitHub-Pull-Request: golang/go#66419
    Reviewed-on: https://go-review.googlesource.com/c/go/+/572956
    TryBot-Result: Gopher Robot <[email protected]>
    LUCI-TryBot-Result: Go LUCI <[email protected]>
    Reviewed-by: David Chase <[email protected]>
    Reviewed-by: Keith Randall <[email protected]>
    Reviewed-by: Emmanuel Odeke <[email protected]>
    Run-TryBot: Emmanuel Odeke <[email protected]>

and

commit 190d0d3e69b113bea0b6b604ba2f0beb62c08741
Author: Brad Fitzpatrick <[email protected]>
Date:   Sat Mar 16 18:29:06 2024 -0700

    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 <[email protected]>
    Reviewed-by: Ian Lance Taylor <[email protected]>
    Reviewed-by: Emmanuel Odeke <[email protected]>
    Reviewed-by: David Chase <[email protected]>
    LUCI-TryBot-Result: Go LUCI <[email protected]>
    Auto-Submit: Brad Fitzpatrick <[email protected]>