Gas estimation issues w/ MultiSend [APE-1294]

[fubuloubu]

@banteg:

still encountering problems when trying to submit a multisend tx. they are mostly coming from gas estimation:

• prepare_transaction tries to estimate gas from a safe address
• it fails when the safe has no ether to cover the gas costs, but in reality safe itself never pays for gas
• it fails because it tries to estimate a call to the multisend contract, while it only supports delegatecall

we need to somehow override the tx handler or find a way to pass additional metadata to skip some checks

Originally posted by @banteg in #11 (comment)