fix(bkgpslog):Reduce logging downtime between buffer rounds

Reduce logging downtime between buffer rounds by moving commands into
the asynchronous `magicWriteBuffer()` function that is spun off
immediately after `gpspipe` has collected data.

Add functionality to `dateTimeShort()` and `dateShort` so the start
time of each saved chunk appended to the `tar` archive can be
calculated indirectly from when `magicWriteBuffer()` starts rather
than performing slow `date` and associated bash variable changes in
the main loop. This start time is needed to name each inserted file as
well as to identify which `tar` file to insert into.

Save gpspipe buffer data `/dev/shm` instead of a Bash variable.

Add `appendFileTar()` function to facilitate reading and processing of
buffer data from `/dev/shm`.