Use fee estimation API when creating vote-for-aggregate-key transactions

[hstove]

During epoch 2.5, signers are required to pay a transaction fee when broadcasting their vote-for-aggregate-key transactions. At the moment, this fee is decided from the field tx_fee_ustx in the signer's config file.

A more robust approach would be to call the node's fee estimation API and use that as the fee.

Some questions:

• Which fee estimate should be used (low/medium/high)? medium
• Should this be configurable (aka opt-in or opt-out)? opt-out
• Should there be a "max fee" option? yes