mgmt: mcumgr: grp: fs_mgmt: Improve upload and download performance

This vastly increases the performance of file transfers using the
fs_mgmt group over MCUmgr by allowing the file handle to remain open
between commands instead of having to open, feek, read/write then
close the file handle for each invocation.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>