proxy+pageserver: shared leaky bucket impl (#8539)

In proxy I switched to a leaky-bucket impl using the GCRA algorithm. I
figured I could share the code with pageserver and remove the
leaky_bucket crate dependency with some very basic tokio timers and
queues for fairness.

The underlying algorithm should be fairly clear how it works from the
comments I have left in the code.

---

In benchmarking pageserver, @problame found that the new implementation
fixes a getpage throughput discontinuity in pageserver under the
`pagebench get-page-latest-lsn` benchmark with the clickbench dataset
(`test_perf_olap.py`).
The discontinuity is that for any of `--num-clients={2,3,4}`, getpage
throughput remains 10k.
With `--num-clients=5` and greater, getpage throughput then jumps to the
configured 20k rate limit.
With the changes in this PR, the discontinuity is gone, and we scale
throughput linearly to `--num-clients` until the configured rate limit.

More context in
https://github.com/neondatabase/cloud/issues/16886#issuecomment-2315257641.

closes https://github.com/neondatabase/cloud/issues/16886

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>
Co-authored-by: Christian Schwarz <christian@neon.tech>

Cargo.lock

@@ -2950,17 +2950,6 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "830d08ce1d1d941e6b30645f1a0eb5643013d835ce3779a5fc208261dbe10f55"
 
-[[package]]
-name = "leaky-bucket"
-version = "1.0.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8eb491abd89e9794d50f93c8db610a29509123e3fbbc9c8c67a528e9391cd853"
-dependencies = [
- "parking_lot 0.12.1",
- "tokio",
- "tracing",
-]
-
 [[package]]
 name = "libc"
 version = "0.2.150"
@@ -3714,7 +3703,6 @@ dependencies = [
  "humantime-serde",
  "hyper 0.14.26",
  "itertools 0.10.5",
- "leaky-bucket",
  "md5",
  "metrics",
  "nix 0.27.1",
@@ -6983,7 +6971,6 @@ dependencies = [
  "humantime",
  "hyper 0.14.26",
  "jsonwebtoken",
- "leaky-bucket",
  "metrics",
  "nix 0.27.1",
  "once_cell",