Add support for pubsub clients

[brson]

Right now we are polling for updates, and for several chains we tend to fall behind and never catch up. Subscribing to new blocks may get more reliable pacing, and may be better for networks that ban us for our high query rate.

This will complicate the code considerably. Have to think about the design some more.

We can start with Solana and Ethers since those are the two protocols where we are having problems.

[brson]

We've improved our block request pacing and should be more reliable now. As long as we are mostly making non-redundant requests and mostly asking for blocks when new blocks are available, it's not obvious that pubsub would be more efficient.

We probably don't need to do this until some chain tells us we must.