test(remoting): unit test

[air-3]

What this PR does:

1. unit test for XidLoadBalance
2. XidLoadBalance bug fix

Which issue(s) this PR fixes:
session is nil, RemoteAddr method cannot be called directly

Fixes #587

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #599 (dd9028c) into master (5835f09) will increase coverage by 0.21%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
+ Coverage   37.03%   37.25%   +0.21%     
==========================================
  Files         175      175              
  Lines       11659    11659              
==========================================
+ Hits         4318     4343      +25     
+ Misses       6977     6952      -25     
  Partials      364      364              

Files Changed	Coverage Δ
pkg/remoting/loadbalance/xid_loadbalance.go	100.00% <100.00%> (+100.00%)	⬆️