Cache DiskArray write header in-memory

[benjaminwinger]

Still a bunch of tests failing.

Additionally, as far as I can tell, the InMemDiskArray isn't actually doing anything meaningful. operator[] is unused (except in the HashIndexBuilder via InMemDiskArrayBuilder) and get/update are just using the BaseDiskArray implementation. The description suggests that it's supposed to cache the entirety of the metadataDA in memory, but I don't think it's done that for a while (hence our attempts to cache metadata locally when updating columns).

The cached header was creating some issues with the InMemDiskArray code, and while I think I was able to fix them in the initial version of this (or at least fix the crashes), being unused the code is not well tested. I've replaced InMemDiskArray with a thin shim for now, as it was doing some unnecessary work reading the pages into memory.

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.95%. Comparing base (4d21128) to head (9e0e4f1).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/storage/storage_structure/disk_array.cpp	88.46%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3109      +/-   ##
==========================================
- Coverage   91.97%   91.95%   -0.03%     
==========================================
  Files        1171     1171              
  Lines       44041    44129      +88     
==========================================
+ Hits        40508    40580      +72     
- Misses       3533     3549      +16     

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