Refactor gun pooling and simplify adapter option insertion

Closes #1834, #1700, and #1680

See merge request !2479

Refactor notification settings

See merge request !2602

Add rejection reason to our MRFs

See merge request !2759

Bring back oban job pruning

Closes #1945

See merge request !2768

Include port in host for signatures

See merge request !2766

Closes #1945

This produced error log messages about GenServer termination
every time the connection was not open due to a timeout.

Instead we stop with `{:shutdown, <gun_error>}` since shutting down
when the connection can't be established is normal behavior.

- `verify_fun` is not useful now
- use `customize_check_hostname` (OTP 20+ so OK)
- `partial_chain` is useless as of OTP 21.1 (wasn't there, but hackney/..
uses it)

The new pooling code just removes the connection when it's down,
there is no need to reconnect a connection that is just sitting idle,
better just open a new one next time it's needed

The numbers of the native time unit were so small the CRF was always 1,
making it an LRU. This commit switches the time to miliseconds and changes
the time delta multiplier to the one yielding mostly highest hit rates according
to the paper

When the application restarts (which happens after certain config
changes), the limiters are not destroyed, so `ConcurrentLimiter.new`
will produce {:error, :existing}

There is no need for streaming the body if there is no body

See https://bugs.erlang.org/browse/ERL-1260 for more info.

The ssl match function is basically copied from mint, except
that `:string.lowercase/1` was replaced by `:string.casefold`.
It was a TODO in mint's code, so might as well do it since we don't need
to support OTP <20.

Closes #1834

`:retry_timeout` and `:retry` got removed because reconnecting on failure is
something the new pool intentionally doesn't do.

`:max_overflow` had to go in favor of `:max_waiting`, I didn't reuse the key because
the settings are very different in their behaviour.

`:checkin_timeout` got removed in favor of `:connection_acquisition_wait`,
I didn't reuse the key because the settings are somewhat different.

I didn't do any migrations/deprecation warnings/changelog entries because
these settings were never in stable.

While running this in production I noticed a number of ghost
processes with all their clients dead before they released the connection,
so let's track them to log it and remove them from clients