experiment: save module info and code cache in background

[dsherret]

Requires: denoland/deno_core#752

Benchmark 3: DENO_DIR=/Users/david/dev/scratch/writable_deno_dir ../deno/deno_old run --cached-only -A demo.ts
  Time (mean ± σ):     203.1 ms ±  35.3 ms    [User: 110.9 ms, System: 101.1 ms]
  Range (min … max):   177.4 ms … 297.3 ms    12 runs
 
Benchmark 4: DENO_DIR=/Users/david/dev/scratch/writable_deno_dir ../deno/target/release/deno run --cached-only -A demo.ts
  Time (mean ± σ):     176.6 ms ±  30.3 ms    [User: 110.3 ms, System: 104.5 ms]
  Range (min … max):   161.1 ms … 285.4 ms    16 runs

Together with WAL:

Benchmark 4: DENO_DIR=/Users/david/dev/scratch/writable_deno_dir ../deno/target/release/deno run --cached-only -A demo.ts
  Time (mean ± σ):     142.7 ms ±   5.9 ms    [User: 110.0 ms, System: 69.3 ms]
  Range (min … max):   135.6 ms … 153.2 ms    19 runs

[dsherret]

Benchmarked against the changes from #23955 and main is now fast enough that this causes more overhead than that (though it would still be good to investigate this future. I think this might improve startup time for scripts that take a while to run, but not for ones that are quick to execute)