app-layer-htp: use stream depth with filestore

app-layer-htp: use stream depth with filestore

This permits to use stream-depth value set for file-store.

Currently if a file is being stored and hits a limit,
such as request or response body, it will be truncated
although file-store.stream-depth is enabled but the file should be
closed and not truncated.

Two unit tests have been added to verify that:
- a file is stored correctly
- chunk's length computation doesn’t cause an underflow