[feat]: Use nproc -X procs for database compaction events

[gStart9]

Prerequisites

• I have searched for existing issues that already suggest this feature.

Describe the Feature Request

Greatly speed up db compaction on SSDs by using multiple CPU cores.

Describe the Use Case

Users with multiple CPU cores and SSDs.
This will greatly speed up compaction as of electrs v0.8.10:
romanz/electrs#1138

Describe Preferred Solution

4 is probably a good number of processor cores to leave available (that leaves at least 2 cores with cache available for all other containers)

Selection of number to set db_parallelism to in electrs.toml something like:

if [[ $(nproc) -gt 4 ]]
then
 electrs_use_procs=$(expr $(nproc) - 4)
else
 electrs_use_procs=1
fi
echo $electrs_use_procs

Equivalent typescript (assuming we can call nproc):

const totalCores = parseInt(await compat.mod("nproc"), 10);
const adjustedCores = totalCores > 4 ? totalCores - 4 : 1;

Describe Alternatives

Alternative is to leave it at the default (1)

Anything else?

No response