wip: rework uid to use bits better

[jeffwashington]

Problem

currently the accounts index uses a u64 per entry to hold a single bit of 'occupied'

Summary of Changes

64 entries share a single bitfield.

Fixes #

[jeffwashington]

index file size decreases by 8 bytes for each allocated entry:
light blue line is this pr
others are various other disk index implementations
The top purple line is when we move a single data entry into the index file (note that this change is not present in that pr).

[codecov]

Codecov Report

Merging #30563 (310350b) into master (4285cb2) will decrease coverage by 0.1%.
The diff coverage is 83.3%.

@@            Coverage Diff            @@
##           master   #30563     +/-   ##
=========================================
- Coverage    81.4%    81.4%   -0.1%     
=========================================
  Files         723      723             
  Lines      203533   203573     +40     
=========================================
+ Hits       165845   165870     +25     
- Misses      37688    37703     +15