Gorse version
0.5.0-alpha.3

Describe the bug
gorse-master fails with the following error:
fatal error: concurrent map read and map write

goroutine 210 [running]:
github.com/zhenghaoz/gorse/logics.(*NonPersonalized).Push(0xc000179270, {{0xc01273d620, 0xe}, 0x0, {0xc0127399b0, 0x3, 0x3}, {0x139c2440, 0xedfe8550d, 0x0}, ...}, ...)
/src/logics/non_personalized.go:155 +0x7bc
github.com/zhenghaoz/gorse/master.(*Master).LoadDataFromDatabase.func2(0xc003a6bf78?, 0xc0001320b8?)
/src/master/tasks.go:1644 +0x78e
github.com/zhenghaoz/gorse/base/parallel.Parallel.func2(0x0)
/src/base/parallel/parallel.go:69 +0xb0
created by github.com/zhenghaoz/gorse/base/parallel.Parallel in goroutine 146
/src/base/parallel/parallel.go:59 +0x127

To Reproduce
Change n_jobs to anything greater than 1, and try running the cluster

Expected behavior
No failure