fix #13529, slowdown with large number of async sleep calls

[JeffBezanson]

The problem was performance degradation of ObjectIdDict with many deleted items. The table needs to be rehashed after a large number of deletions.

[vtjnash]

can we return a Nullable or flag or something, rather than having one particular sentinel value in the system that can't be put into an ObjectIdDict without causing bugs?

[JeffBezanson]

I think the only really efficient alternative is to return NULL. Would you be comfortable with this:

    ptr = ccall(:jl_eqtable_pop, Ptr{Void}, (Any, Any, Any), t.ht, key)
    if ptr == C_NULL
        return default
    end
    val = unsafe_pointer_to_objref(ptr)

or is it too risky due to the temporary lack of a root for the result?

[vtjnash]

how about:

val = ccall(:jl_eqtable_pop, Any, (Any, Any, Any), t.ht, key, default)
val === default || (t.ndel += 1) # this can underestimate `ndel`, but that's ok
return val

it'll be nice when Oscar makes his PR for stack allocating these, so that the obvious solution (return a tuple (val, found)) is also the most efficient

[JeffBezanson]

That doesn't fix 2-argument pop! below, which could still incorrectly throw an error.

[vtjnash]

yes, but that's an existing failure