Channels can now block depending on maxItems (#6153)

tests/parallel/tblocking_channel.nim

@@ -0,0 +1,37 @@
+discard """
+output: ""
+"""
+import threadpool, os
+
+var chan: Channel[int]
+
+chan.open(2)
+chan.send(1)
+chan.send(2)
+doAssert(not chan.trySend(3)) # At this point chan is at max capacity
+
+proc receiver() =
+    doAssert(chan.recv() == 1)
+    doAssert(chan.recv() == 2)
+    doAssert(chan.recv() == 3)
+    doAssert(chan.recv() == 4)
+    doAssert(chan.recv() == 5)
+
+var msgSent = false
+
+proc emitter() =
+    chan.send(3)
+    msgSent = true
+
+spawn emitter()
+# At this point emitter should be stuck in `send`
+sleep(100) # Sleep a bit to ensure that it is still stuck
+doAssert(not msgSent)
+
+spawn receiver()
+sleep(100) # Sleep a bit to let receicer consume the messages
+doAssert(msgSent) # Sender should be unblocked
+
+doAssert(chan.trySend(4))
+chan.send(5)
+sync()