Don't set the timer when inactivity == infinity
Note about 'wait'
Rename DEFAULT_REQUESTS to CONCURRENT_REQUESTS

This value is not configurable so the prefix might've been misleading.
Group configurable options together
Preliminary work to support acknowledgements

Supporting `wait` needs to be done before as it will incorporate
identifying requests by RID. Acks also need to identify requests.
Send empty <body> elements instead of empty cdatas

This is compliant with the XEP.