Fixes for flywire_cave_query timestamp handling

[jefferis]

• Was not possible to provide a timestamp
• Also adds support for using expired materialisation versions (via their respective timestamps)

[codecov]

Codecov Report

Merging #159 (5d39c5e) into master (d907d3b) will decrease coverage by 1.03%.
The diff coverage is 82.14%.

❗ Current head 5d39c5e differs from pull request most recent head 021c18b. Consider uploading reports for the commit 021c18b to get more accurate results

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   45.42%   44.38%   -1.04%     
==========================================
  Files          32       32              
  Lines        3868     3893      +25     
==========================================
- Hits         1757     1728      -29     
- Misses       2111     2165      +54     

Impacted Files	Coverage Δ
R/cave.R	60.16% <82.14%> (+10.16%)	⬆️
R/autosyn.R	36.59% <0.00%> (-9.39%)	⬇️
R/utils.R	47.90% <0.00%> (-1.61%)	⬇️

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