perf: have the changes functions return an iter

[josephschorr]

This should reduce memory usage a bit when reading the changes, as it can compute one-at-a-time

[codecov]

Codecov Report

❌ Patch coverage is 64.10256% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.68%. Comparing base (48ae087) to head (7fbb0b6).

Files with missing lines	Patch %	Lines
internal/datastore/mysql/watch.go	42.31%	13 Missing and 2 partials ⚠️
internal/datastore/memdb/memdb.go	47.06%	6 Missing and 3 partials ⚠️
internal/datastore/spanner/watch.go	0.00%	6 Missing ⚠️
internal/datastore/common/changes.go	92.16%	3 Missing and 1 partial ⚠️
internal/datastore/crdb/watch.go	60.00%	3 Missing and 1 partial ⚠️
internal/datastore/postgres/watch.go	42.86%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2578      +/-   ##
==========================================
- Coverage   77.71%   77.68%   -0.02%     
==========================================
  Files         441      441              
  Lines       54378    54391      +13     
==========================================
- Hits        42254    42249       -5     
- Misses       9505     9520      +15     
- Partials     2619     2622       +3     

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[tstirrat15]

Changes look good to me. Is there any reason not to do this? Is it worth benchmarking?