Provide a way to highlight the location of slow locks

[IanButterworth]

Replaces #55120

Without something like this, it can be quite tricky to figure out where slow locks are happening.

julia> Base.slow_lock_warn = 1
1

julia> @sync begin
       l = ReentrantLock()
       @async begin
           lock(l) do
               sleep(2)
           end
       end
       @async begin
           lock(l) do
               sleep(2)
           end
       end
       end
┌ Warning: Taking longer than 1s to get a lock
│   exception =
│    Lock contention warning
│    Stacktrace:
│     [1] (::Base.var"#slowlock#lock##14")(rl::ReentrantLock)
│       @ Base ./REPL[8]:14
│     [2] lock
│       @ ./REPL[8]:2 [inlined]
│     [3] lock(f::var"#47#48", l::ReentrantLock)
│       @ Base ./lock.jl:262
│     [4] (::var"#45#46"{ReentrantLock})()
│       @ Main ./REPL[9]:9
└ @ Base REPL[8]:16

[Seelengrab]

If julia had a debug vs. release mode, this would be a very good candidate for only enabling in the former :/ Taking a backtrace and spawning a Timer are quite heavy..

[IanButterworth]

It's disabled by default. Note the -1

[Seelengrab]

Yes, but the code for the Timer still has to be there & visible for effects & inference, since the global is not a constant. I'd also be very anxious around some other piece of code enabling that out of perceived convenience for their code, not realizing that this could introduce unwanted prints/logs in some other library. That's not something that can happen with a true debug vs. release distinction on the compiler level :)