fix: race condition in txConns

[teckick]

use txConns in Executor without locking can cause race condition, which leads to gaea-proxy panic (#50)

fix this problem by adding lock to txConns.

[codecov]

Codecov Report

Merging #51 into master will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   53.68%   53.66%   -0.03%     
==========================================
  Files         135      135              
  Lines       25983    25993      +10     
==========================================
  Hits        13949    13949              
- Misses      11060    11070      +10     
  Partials      974      974              

Flag	Coverage Δ
#unittests	53.66% <0.00%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
proxy/server/executor.go	1.34% <0.00%> (-0.03%)	⬇️
proxy/server/executor_handle.go	0.00% <0.00%> (ø)

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[chicliz]

LGTM