Avoid adding more than one MWI subscribe operation to the command channel

Ignore-this: 758735e58a77a28329e7777943fc2fe9

darcs-hash:20100909194059-56b4d-b118383be9e46063c32f234b8f4c942a48b50a3f
Properly fix race condition when first NOTIFY for MWI arrives

Ignore-this: cf11e87e2a1462c8d66c178e608a68

darcs-hash:20100910125556-56b4d-ed11a0216d1acaf0a77fb7d9c0210dde03f02948