[off-chain worker] expose a transaction based storage API

[drahnr]

As suggested in #6010 https://github.com/paritytech/substrate/pull/6010/files/ce606550a9e118772858307020edbf7bcdf0daa1#r424727256 for sake of data consistency an additional API based on Transactions might be advantageous.

let mut tx = Transaction::new();
tx.clear(key1);
tx.set(key2, value);
offchain::commit(tx);

Using this instead of multiple API calls have also potential storage performance benefits.