Move `GetElastiCacheEndpoints{Timeout}` to `aws`.
`EmceeStore`: Remove `parseTimeout()`.
`EmceeStore`: `ia` => `ChannelIDs`.

`fetchAppIDArray` now releases the memcached client before removing duplicate
channel IDs.
Simplify type assertions and `recover()` calls.
`ValidUAID`: Add an `isHex` helper.
`EmceeStore`: `cr` => `ChannelRecord`.
`EmceeStore`: Remove `MinConns` and `FreeClient`.

`EmceeStore.Init()` no longer seeds the pool with an initial number of
connections, but opens a single connection to ensure the settings are
valid.