Allow multiple transfers in a single tx

[grarco]

We should allow multiple token transfers in a single transaction, i.e. involving more than one sender/receiver and with different tokens. We could either modify the current tx_transfer or come up with a different one. We'll also need to update the client and SDK to produce this new type of transactions.

[cwgoes]

It may make sense to make this change in conjunction with #1677.

[cwgoes]

Moving this to phase 1 so we can avoid breaking transaction format changes.

[cwgoes]

Note that this is two parts:

• Batching multiple inner transactions in one transaction (this is Tx batching #3103)
• Vectorized balance changes in a single transfer inner tx (no PR yet)