Our pruning of session tokens from Redis is not perfect because we can
only delete tokens that are expired-but-not-yet-pruned-from-MySQL. This
leaves us with some number of zombie session tokens that are lying
around in Redis, the effect of which could be to sometimes show
incorrect session information in the device manager (albeit with very
low probability).
To eliminate that possbility, this change speculatively deletes from
Redis when creating the session token. In addition, the maximum number
of Redis connections is bumped up from 100 to 200, because we can expect
the number of concurrent Redis operations to increase significantly.