[http-server] add exchangeId lock

[mistermoe]

lock on exchangeId. should only be able to add 1 message to a given exchange at any time

[diehuxx]

Is it possible to do this at the http-server level? Since ExchangeApi only deals with getting messages, I interpreted that the callback is what should save a given message. If that's the case, it's up to the implementer of the callback to manage a lock.

[diehuxx]

Addressed by #168. Though my previous comment still stands. The consumer of http-server must store the messages in an exchange by leveraging callbacks and ExchangeApi. Assuming the consumer has stored messages and exposed them to http-server via ExchangeApi, the Exchange state machine in #168 will handle the rest.

Disregard, I misread the intent of this issue.