Add test: single write size lower than segment size
Add test: single write size lower than segment size

For now at least, single writes that are greater then the segment size are
not allowed.

There are two ways for ensuring that a payload greater than the segment size
gets succcessfully written:
1. The payload gets split into multiple log frames and then those get saved
   into multiple segments.
   To ensure 'zero-copy' when reading the frame, the client has to read multple
   frames and then re-assemble them to recreate the original payload.
2. A new segment is created, with a size great enough the accomodate
   the payload.

rbruggem/mqlog,rbruggem/mqlog