tokio-epoll-uring: retry on launch failures due to locked memory (#7141)

refs https://github.com/neondatabase/neon/issues/7136

Problem
-------

Before this PR, we were using
`tokio_epoll_uring::thread_local_system()`,
which panics on tokio_epoll_uring::System::launch() failure

As we've learned in [the

past](https://github.com/neondatabase/neon/issues/6373#issuecomment-1905814391),
some older Linux kernels account io_uring instances as locked memory.

And while we've raised the limit in prod considerably, we did hit it
once on 2024-03-11 16:30 UTC.
That was after we enabled tokio-epoll-uring fleet-wide, but before
we had shipped release-5090 (c6ed86d3d0)
which did away with the last mass-creation of tokio-epoll-uring
instances as per

    commit 3da410c8fe
    Author: Christian Schwarz <christian@neon.tech>
    Date:   Tue Mar 5 10:03:54 2024 +0100

tokio-epoll-uring: use it on the layer-creating code paths (#6378)

Nonetheless, it highlighted that panicking in this situation is probably
not ideal, as it can leave the pageserver process in a semi-broken
state.

Further, due to low sampling rate of Prometheus metrics, we don't know
much about the circumstances of this failure instance.

Solution
--------

This PR implements a custom thread_local_system() that is
pageserver-aware
and will do the following on failure:
- dump relevant stats to `tracing!`, hopefully they will be useful to
  understand the circumstances better
- if it's the locked memory failure (or any other ENOMEM): abort() the
  process
- if it's ENOMEM, retry with exponential back-off, capped at 3s.
- add metric counters so we can create an alert

This makes sense in the production environment where we know that
_usually_, there's ample locked memory allowance available, and we know
the failure rate is rare.

clippy.toml

@@ -2,6 +2,8 @@ disallowed-methods = [
     "tokio::task::block_in_place",
     # Allow this for now, to deny it later once we stop using Handle::block_on completely
     # "tokio::runtime::Handle::block_on",
+    # use tokio_epoll_uring_ext instead
+    "tokio_epoll_uring::thread_local_system",
 ]
 
 disallowed-macros = [