[Issue 362] Improve Performance constructing the asofJoin()

[tnixon]

Fix for #362 - improving performance of constructing the asofJoin(...) function.

[codecov]

Codecov Report

Merging #363 (8cd949f) into master (9f2bbe4) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 8cd949f differs from pull request most recent head 78fc76e. Consider uploading reports for the commit 78fc76e to get more accurate results

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
- Coverage   87.72%   87.67%   -0.06%     
==========================================
  Files           6        6              
  Lines         872      876       +4     
  Branches      177      182       +5     
==========================================
+ Hits          765      768       +3     
  Misses         81       81              
- Partials       26       27       +1     

Files Changed	Coverage Δ
python/tempo/tsdf.py	86.32% <100.00%> (-0.11%)	⬇️

[rportilla-databricks]

LGTM

[R7L208]

I suggested a few changes that make the code simpler. I'd like to move away from lambda unless it's actually needed

[R7L208]

NIT but suggested adding TODO on the comment regarding batch renaming so we can find it again easily.

Approving though. Thanks for the updates