fix cleanup after update bug and concurrent writes to allSnapshots

[alexhornbake]

Changes

• Add Mutex to protect writes to allSnapshots related to Make snapshots concurrency safe #45
• Differentiate update and create snapshot so that snapshot.evaluated bool can be correctly set during update.

Prior to this change, cleanup after an update would cause removal of the update ie:

=== RUN   TestCleanupUpdate
Updating snapshot `1`
Removing unused snapshot `2`

This manifested in a project when running:
go test -v ./some_package/... -- -u

[sjkaliski]

Thanks @alexhornbake!