Ensure progress metric is updated frequently during basebackup [BF-956]

[alexole]

Adapt the fix from #549

[codecov[bot]]

Codecov Report

Merging #550 (a437667) into main (370b1f3) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #550      +/-   ##
==========================================
- Coverage   90.76%   90.72%   -0.05%     
==========================================
  Files          30       30              
  Lines        4593     4594       +1     
==========================================
- Hits         4169     4168       -1     
- Misses        424      426       +2     

Impacted Files	Coverage Δ
pghoard/basebackup/chunks.py	96.79% <100.00%> (+0.02%)	:arrow_up:
pghoard/webserver.py	89.12% <0.00%> (-0.95%)	:arrow_down:
pghoard/basebackup/base.py	85.35% <0.00%> (-0.23%)	:arrow_down:
pghoard/pghoard.py	83.05% <0.00%> (+0.33%)	:arrow_up:
pghoard/walreceiver.py	97.85% <0.00%> (+0.71%)	:arrow_up:

[alexole]

test_pause_on_disk_full seems to be absolutely broken now.

[alexole]

I think you can merge it, there is still one flaky test_pause_on_disk_full