Implement user operation bundling on selected chains

[TheDivic]

What?

Implement bundling of user operations on selected chains that have the most volume in our system.

Why?

Currently we don't bundle, we just relay one user operation per transaction. If we want to conform to ERC-4337 standards we should start bundling, and it will probably also save us money and gas on high-volume transactions.

How?

To do this we need the following:

• A bundler private "mempool" implementation (currently we just have a FIFO queue, powered by RabbitMQ)
• A strategy to pick and bundle user operations
• A way to handle low-volume -- meaning no one should wait for too long for their user op to be included in a bundle if there are no other user ops to fill the bundle with. If it's not relayed within Xms we should just relay it in a standalone transaction containing only one user op.