list index refractor

[damotiansheng]

list底层索引改为Btree，支持自动化merge

[codecov]

Codecov Report

Merging #467 (dbc0e0b) into master (a1a81e6) will increase coverage by 0.21%.
The diff coverage is 95.51%.

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
+ Coverage   73.71%   73.92%   +0.21%     
==========================================
  Files          31       31              
  Lines        4420     4487      +67     
==========================================
+ Hits         3258     3317      +59     
- Misses        887      893       +6     
- Partials      275      277       +2     

Files Changed	Coverage Δ
btree.go	93.25% <85.71%> (-1.41%)	⬇️
list.go	74.25% <94.38%> (+0.58%)	⬆️
tx.go	81.78% <100.00%> (+0.63%)	⬆️
tx_list.go	70.98% <100.00%> (-1.18%)	⬇️
utils.go	76.31% <100.00%> (+6.99%)	⬆️

... and 1 file with indirect coverage changes