Make Allow middleware thread-safe without duping

[liveh2o]

The initial Allow middleware implementation was not thread-safe. A "temporary" (eight years ago!) hack was put in place that simply duped the middleware stack to ensure threadsafety. Not only is this a hack, but it also adds unnecessary overhead to each request since each dup generated more objects that would need to be GC'd.

This change makes the Allow middleware thread-safe without duping, and yields a whopping 85x (!) increase in performance (via bin/benchmark):

Before:

ruby 2.7.6p219 (2022-04-12 revision c9c2245c0a) [arm64-darwin22]
Warming up --------------------------------------
    Hit    2.318k i/100ms
    Miss   2.317k i/100ms
Calculating -------------------------------------
    Hit   22.267k (± 2.8%) i/s (44.91 μs/i) - 111.264k in 5.000717s
    Miss  22.266k (± 1.2%) i/s (44.91 μs/i) - 113.533k in 5.099622s

After:

ruby 2.7.6p219 (2022-04-12 revision c9c2245c0a) [arm64-darwin22]
Warming up --------------------------------------
    Hit  188.921k i/100ms
    Miss 186.268k i/100ms
Calculating -------------------------------------
    Hit    1.859M (± 0.5%) i/s (537.92 ns/i) - 9.446M in 5.081332s
    Miss   1.860M (± 1.1%) i/s (537.70 ns/i) - 9.313M in 5.008448s

This implementation uses a simplified Config class that is no longer backed by a hash. This required dropping support for passing config options to the middleware, but that likely was mostly used in tests.