fix race condition in foundRefs.claim()

[igorwwwwwwwwwwwwwwwwwwww]

On re-restarting hound after the initial indexing, I got this:

fatal error: concurrent map writes

goroutine 49 [running]:
runtime.throw(0x14ae4f5, 0x15)
	/usr/local/Cellar/go/1.13.4/libexec/src/runtime/panic.go:774 +0x72 fp=0xc0001bbd78 sp=0xc0001bbd48 pc=0x102fb32
runtime.mapassign_fast64ptr(0x141cd80, 0xc000464060, 0xc000268e60, 0xc0004ee7e0)
	/usr/local/Cellar/go/1.13.4/libexec/src/runtime/map_fast64.go:191 +0x369 fp=0xc0001bbdb8 sp=0xc0001bbd78 pc=0x1013829
github.com/hound-search/hound/searcher.(*foundRefs).claim(...)
	/Users/igor/go/src/github.com/hound-search/hound/searcher/searcher.go:88
github.com/hound-search/hound/searcher.newSearcher(0xc000190580, 0x16, 0xc000168600, 0x22, 0xc00016a0a0, 0xc00044ee60, 0xc0004022a0, 0x657461677261662f, 0x20200a227469672e, 0x2020200a2c7d2020)
	/Users/igor/go/src/github.com/hound-search/hound/searcher/searcher.go:419 +0x356 fp=0xc0001bbee0 sp=0xc0001bbdb8 pc=0x13768d6
github.com/hound-search/hound/searcher.newSearcherConcurrent(0xc000190580, 0x16, 0xc000168600, 0x22, 0xc00016a0a0, 0xc00044ee60, 0xc0004022a0, 0xc00041aea0)
	/Users/igor/go/src/github.com/hound-search/hound/searcher/searcher.go:501 +0xe5 fp=0xc0001bbfa0 sp=0xc0001bbee0 pc=0x1376d75
runtime.goexit()
	/usr/local/Cellar/go/1.13.4/libexec/src/runtime/asm_amd64.s:1357 +0x1 fp=0xc0001bbfa8 sp=0xc0001bbfa0 pc=0x105ce31
created by github.com/hound-search/hound/searcher.MakeAll
	/Users/igor/go/src/github.com/hound-search/hound/searcher/searcher.go:296 +0x1c3

Adding this mutex seems to do the trick, but there might be a better higher-level fix too.