Geth SBAT optionally not use a persisted discovery table

[kevjue]

Expected Behavior

Geth SBAT have an option to not use a persisted discovery table.

Current Behavior

Geth currently always reads and saves the discovery table to a leveldb instance. This can be an issue for cases where a celo network has nodes that will have frequent changing ip addresses (e.g. in an internal kubernetes testnet).