Fix concurrency write map panic

[winwill2012]

Fix concurrency write map panic:

fatal error: concurrent map read and map write

goroutine 163 [running]:
runtime.throw({0x109be48, 0xf5ae00})
/usr/local/go/src/runtime/panic.go:1198 +0x71 fp=0xc0041e0630 sp=0xc0041e0600 pc=0x435bf1
runtime.mapaccess2_faststr(0xc003c2e5e8, 0x26, {0x107e883, 0x7})
/usr/local/go/src/runtime/map_faststr.go:116 +0x3d4 fp=0xc0041e0698 sp=0xc0041e0630 pc=0x412db4
github.com/zhenghaoz/gorse/worker.(*Worker).exploreRecommend(0xc00025e000, {0xc00bad2c00, 0x28, 0x28}, 0x1089476, {0xc000f2ba00, 0xf})
/Users/mac/code/gorse/worker/worker.go:713 +0x90 fp=0xc0041e07a0 sp=0xc0041e0698 pc=0xe68350
github.com/zhenghaoz/gorse/worker.(*Worker).Recommend.func2(0x0, 0x7ffb)
/Users/mac/code/gorse/worker/worker.go:602 +0x2e35 fp=0xc0041e1f48 sp=0xc0041e07a0 pc=0xe65c95
github.com/zhenghaoz/gorse/base.Parallel.func2(0x0)
/Users/mac/code/gorse/base/parallel.go:67 +0xc4 fp=0xc0041e1fc8 sp=0xc0041e1f48 pc=0x78e824
github.com/zhenghaoz/gorse/base.Parallel·dwrap·1()
/Users/mac/code/gorse/base/parallel.go:72 +0x2d fp=0xc0041e1fe0 sp=0xc0041e1fc8 pc=0x78e72d
runtime.goexit()
/usr/local/go/src/runtime/asm_amd64.s:1581 +0x1 fp=0xc0041e1fe8 sp=0xc0041e1fe0 pc=0x468401
created by github.com/zhenghaoz/gorse/base.Parallel
/Users/mac/code/gorse/base/parallel.go:57 +0x130

[codecov-commenter]

Codecov Report

Merging #338 (31a7018) into master (d7d9d35) will decrease coverage by 0.05%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   72.99%   72.94%   -0.06%     
==========================================
  Files          44       44              
  Lines        9109     9118       +9     
==========================================
+ Hits         6649     6651       +2     
- Misses       1752     1756       +4     
- Partials      708      711       +3     

Impacted Files	Coverage Δ
config/config.go	93.87% <77.77%> (-1.05%)	⬇️
worker/worker.go	62.75% <100.00%> (ø)
master/task_manager.go	88.23% <0.00%> (-3.68%)	⬇️

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[zhangzhenghao]

Thanks for your contribution 😁