netutil: add little AwaitReachable func

Change-Id: I52af3add776cd57ca91c073e0ec563c50923d40f
mongo: test using Docker

Under a second to start and test against a real MongoDB instace in Docker:

$ go test -v camlistore.org/pkg/sorted/mongo
=== RUN TestMongoKV
--- PASS: TestMongoKV (0.79 seconds)
PASS
ok      camlistore.org/pkg/sorted/mongo 0.791s

Test is skipped in short mode, or if docker isn't available.

Change-Id: I6f3acc96357cf9c50054b3df6727661c9329e6bf