Improve HTTP event processing

[jfk9w]

Problem

Events submitted via HTTP (/api/v1/s2s/event, for example) currently are buffered in memory (as in "the whole request body is read into a byte slice"). This can cause memory overflow.

Solution

Implement parsing the request body without buffering. It would also be nice to log the first 1000 symbols of the request body in case of errors – see server/middleware/logging.go:22 (some kind of io.Reader implementation is needed which would buffer the first 1000 symbols for logging and concatenate the buffer with the rest of the body when parsing the body). Maybe even write it to a separate log file.