internal/jsonrpc2_v2: fix a racy map assignment in readIncoming

If a call request comes it at the same time that a response becomes
ready for another call (either via an explicit Respond call or the
return from a Handle call), the map read and deletion for the response
could race with the map assignment for the new (incoming) call.

Change-Id: Ifd81dbc490486251e81c7eeae438806356f514af
Reviewed-on: https://go-review.googlesource.com/c/tools/+/388595
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

internal/jsonrpc2/jsonrpc2_test.go

@@ -60,6 +60,14 @@ var callTests = []invoker{
 		notify{"unblock", "a"},
 		collect{"a", true, false},
 	}},
+	sequence{"concurrent", []invoker{
+		async{"a", "fork", "a"},
+		notify{"unblock", "a"},
+		async{"b", "fork", "b"},
+		notify{"unblock", "b"},
+		collect{"a", true, false},
+		collect{"b", true, false},
+	}},
 }
 
 type binder struct {