Adds locking of the credentials store for concurrency safety. It's
important to hold the lock from read -> write so credentials are not
dropped during concurrent writes.

I opted not to attach the lock to the store itself. Instead, I return
the lock on read and require it on write to encourage safe use. Maybe
attaching the source path to the store struct and adding a `lock(&self)`
method would make sense? but then you can forget to take the lock at the
right time. The main problem with the interface here is to write a _new_
store you have to take the lock yourself, and you could make a mistake
by taking a lock for the wrong path or something. The fix for that would
be to introduce a new `CredentialStoreHandle` type or something, but
that seems overzealous rn. We also don't eagerly drop the lock on token
read, although we could.