fix: concurrent write access

[Nazdroth]

While using bigcache today I encountered this error:

fatal error: concurrent map writes
goroutine 2487 [running]:
runtime.throw(0xe697aa, 0x15)
	/usr/local/go/src/runtime/panic.go:774 +0x72 fp=0xc5878dd388 sp=0xc5878dd358 pc=0x4310c2
runtime.mapassign_fast64(0xd1eec0, 0xc347b05bf0, 0x44b94351220b245d, 0xc5880a8000)
	/usr/local/go/src/runtime/map_fast64.go:176 +0x337 fp=0xc5878dd3c8 sp=0xc5878dd388 pc=0x413897
github.com/allegro/bigcache/v2.(*cacheShard).hit(0xc349c61560, 0x44b94351220b245d)

This bug was introduced by me 2 month ago instead of using Lock I used Rlock

[janisz]

@Nazdroth thanks, how about enabling stats in test to prevent this bug in the future?

https://github.com/allegro/bigcache/blob/master/bigcache_test.go#L405-L406

[siennathesane]

I'm going to merge this change now to get the fix out, but I do agree stats should be enabled in the test suite.