Allow synchronous writes from the HTTP endpoint

If the query-string specifies the flag "sync", then wait until all writes have actually occurred before returning.

Based on [this mailing list discussion](https://groups.google.com/d/msg/opentsdb/1gcuDmbaLhE/VL69lq9GTWIJ).

If anyone would like to suggest some pointers for writing tests to go along with this, I'd be happy to update.

I've tested this change against my own client where I was seeing the problem, and this appears to fix it.