update metadata corresponding to spec edits (#2282)

* spec update: metadata keys are lowercase

* add batch parameter to metadata batches

* fix: connecting clients receive METADATA, not RPL_KEYVALUE

* spec update: send RPL_METADATASUBS in a metadata-subs batch

* move some helpers

* bump irctest to forked hash

This is https://github.com/progval/irctest/pull/314 but I don't want to
couple the merges

* fix: empty value is valid

* fix: deleting a nonexistent key gets a FAIL

irc/metadata_test.go

@@ -7,9 +7,12 @@ func TestKeyCheck(t *testing.T) {
 		input  string
 		isEvil bool
 	}{
-		{"ImNormal", false},
+		{"ImNormalButIHaveCaps", true},
+		{"imnormalandidonthavecaps", false},
+		{"ergo.chat/vendor-extension", false},
 		{"", true},
 		{":imevil", true},
+		{"im:evil", true},
 		{"key£with$not%allowed^chars", true},
 		{"key.thats_completely/normal-and.fine", false},
 	}