Add a progress bar indicator for record processing

[pablogsal]

As record processing is an operation that can take an arbitrary amount
of time depending on the size of the file, some users can be frustrated
because they don't know how much time the processing is going to take.

To help with this, add a progress bar indicator that periodically
updates while we are processing records in the FileReader. The class is
updated strategically to avoid impacting the performance of the actual
processing.

[gaborbernat]

Needs a news entry 👍

News entry added

[godlygeek]

I played with this for a while locally. I've pushed some fixup commits that manage to make it much faster, especially in the incomplete file / OOM killer case. Rather than letting Rich decide when to redraw the screen, my version calculates that ourselves, using calls to clock_gettime (which is in the VDSO) rather than making calls into Python code. This winds up being substantially faster, especially when a too-low value was chosen for _update_interval.

Incidentally, I also raised _update_interval from 200 to 100,000. On my WSL box with its relatively slow disk IO, we still process approximately 7,000,000 allocations per second, so at 10 refreshes per second we process ~700k allocations per refresh, which works out to 7 calls to clock_gettime per refresh, which seems like a fair enough price to pay for snappier updates of the progress bar.

[godlygeek]

LGTM