Add option to send each POST with a maximum byte size

[jbeemster]

This will build a POST up to the maximum byte size specified to ensure minimal amount of requests are sent.

This could work well with a combination of this ticket: snowplow/snowplow-ios-tracker#237

[mscwilson]

There's another old issue about payload size - #168 "Add batching based on payload size". Therefore I'm reading this issue as being about oversized payloads - giving users the ability to set a maximum byte size for a single POST request.

An oversize payload can arise from a single Event being large, normally due to a large number of attached context entities. Alternatively, calling Emitter.flushBuffer() could cause a large number of normal size events to be batched into a single payload.

The Android tracker has long had the ability to detect oversize payloads, although I'm not sure what it does differently except logging.