event: do buffering from stream inside the event structure.

Most drivers are significantly worse than this, but I would like to avoid
this double buffering. That is difficult without incurring multiple
syscalls to perform read() on the underlying buffer. Probably worth
testing buffered vs unbuffered mongoc_stream_t instances. The syscall
overhead for reading the 16 byte message header might be less than the
memcpy() of average size buffers.