Fix a JET error around matching methods for push!(...) (#173)

* Fix a JET error around matching methods for `push!(...)`

```
┌  @ Distributed /workpath/Distributed.jl/src/process_messages.jl:387
│ no matching method found `push!(::Nothing, ::Any)` (1/2 union split): Distributed.push!(default_worker_pool()::Union{Nothing, Distributed.AbstractWorkerPool}, w.id::Any)
└────────────────────
```

(cherry picked from commit f15f306adaafbedc2f431c96087da2ea483c9558)

* Add a `::AbstractWorkerPool` return type annotation to `default_worker_pool()`

* Add a type annotation in an additional place, to fix another JET error

(cherry picked from commit b7c43b235a963d19ec1c5e5aa9b8a565b6e49c34)

Author: DilumAluthge

src/workerpool.jl

@@ -289,7 +289,7 @@ julia> default_worker_pool()
 WorkerPool(Channel{Int64}(sz_max:9223372036854775807,sz_curr:3), Set([4, 2, 3]), RemoteChannel{Channel{Any}}(1, 1, 4))
 ```
 """
-function default_worker_pool()
+function default_worker_pool()::AbstractWorkerPool
     # On workers retrieve the default worker pool from the master when accessed
     # for the first time
     if _default_worker_pool[] === nothing
@@ -299,7 +299,7 @@ function default_worker_pool()
             _default_worker_pool[] = remotecall_fetch(()->default_worker_pool(), 1)
         end
     end
-    return _default_worker_pool[]
+    return _default_worker_pool[]::AbstractWorkerPool
 end
 
 """