Make snapshots concurrency safe

[alexhornbake]

I may tackle this one, but want to raise here first.

I think it would be worth converting snapshots map[snapshotID]*snapshot to use sync.Map, or perhaps just use a mutex to protect it. Wondering if there is a preference for minimum golang version that should be targeted, since sync.Map was introduced in golang 1.9. Thoughts?

For context, my use case is a set of tests that assert snapshots from multiple concurrent goroutines. As written, it hits the error: fatal error: concurrent map writes while updating snapshots on github.com/beme/abide/abide.go:230.

[sjkaliski]

@alexhornbake go for it! 1.9 is a sufficient minimum version.

[sjkaliski]

@alexhornbake thoughts? Should be ok now that we dropped 1.8.x support.

[sjkaliski]

@alexhornbake good to close?

[alexhornbake]

@sjkaliski yup!