Add redis or similar scalable backend

[marcopeereboom]

We want to make dcrtime cloudhyperscale web 3.5 so please add a redis or similar cloud key/value store
as a backend.

[sefbkn]

What is the driving use case, and what kind of data needs be stored for how long?

I'm guessing the need to sync/sort unanchored hashes across multiple servers, possibly cache old trees to calculate the paths w/ an expiry, and coordinate which server writes the root to the blockchain.

Looking to clarify the scope of this task.

[amass01]

As discussed on matrix we decided to to go with postgres, see full discussion here:

https://matrix.to/#/!mhdHEMEQocnqUqtuYD:decred.org/$Hia4hngVJnL3Za4Bwtfl6mAUe1UjnWXGmNMZVkVBZOc?via=decred.org&via=matrix.org&via=zettaport.com