This creates an interface for TokenStore. The main motivation of this right
now is testing, since I'm punting on figuring out a mock framework right
now (due to time constraints).
* src/server/token/TokenStore.ts: Rename file.
* src/server/token/store/PersistentTokenStore.ts: Rename from TokenStore.
* test/server/token/TokenStoreTest.ts: Rename file.
* test/server/token/store/PersistentTokenStoreTest.ts: Rename from TokenStoreTest.
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