Use wkspawn from ConcurrentUtilities instead of Threads.spawn (#469)

Should fix #336.

For more context, see the [same
fix](JuliaData/CSV.jl@077e177)
we made for this in CSV.jl.

Basically, objects interpolated into or returned from spawned tasks can
be unexpectedly kept alive long after the task has finished and the
object should have been garbage-collected due to individual threads
holding the most recent task as a reference. Using `@wkspawn` ensures
tasks themselves don't hold references to any of these once they're done
executing.

Author: quinnj

src/append.jl

@@ -169,7 +169,7 @@ function append(
     # start message writing from channel
     threaded = ntasks > 1
     tsk =
-        threaded ? (Threads.@spawn for msg in msgs
+        threaded ? (@wkspawn for msg in msgs
             Base.write(io, msg, blocks, sch, alignment)
         end) : (@async for msg in msgs
             Base.write(io, msg, blocks, sch, alignment)
@@ -191,7 +191,7 @@ function append(
         end
 
         if threaded
-            Threads.@spawn process_partition(
+            @wkspawn process_partition(
                 tbl_cols,
                 dictencodings,
                 largelists,

src/table.jl

@@ -419,7 +419,7 @@ function Table(blobs::Vector{ArrowBlob}; convert::Bool=true)
     dictencoded = Dict{Int64,Meta.Field}() # dictionary id => field
     sync = OrderedSynchronizer()
     tsks = Channel{Any}(Inf)
-    tsk = Threads.@spawn begin
+    tsk = @wkspawn begin
         i = 1
         for cols in tsks
             if i == 1
@@ -522,7 +522,7 @@ function Table(blobs::Vector{ArrowBlob}; convert::Bool=true)
             elseif header isa Meta.RecordBatch
                 anyrecordbatches = true
                 @debugv 1 "parsing record batch message: compression = $(header.compression)"
-                Threads.@spawn begin
+                @wkspawn begin
                     cols = collect(VectorIterator(sch, $batch, dictencodings, convert))
                     put!(() -> put!(tsks, cols), sync, $(rbi))
                 end

src/write.jl

@@ -167,7 +167,7 @@ function Base.open(
     # start message writing from channel
     threaded = Threads.nthreads() > 1
     task =
-        threaded ? (Threads.@spawn for msg in msgs
+        threaded ? (@wkspawn for msg in msgs
             Base.write(io, msg, blocks, schema, alignment)
         end) : (@async for msg in msgs
             Base.write(io, msg, blocks, schema, alignment)
@@ -296,7 +296,7 @@ function write(writer::Writer, source)
             put!(writer.msgs, recbatchmsg)
         else
             if writer.threaded
-                Threads.@spawn process_partition(
+                @wkspawn process_partition(
                     tblcols,
                     writer.dictencodings,
                     writer.largelists,