Abstract SCB handling into standalone lib

[absurdfarce]

Consider pulling Astra SCB logic in cql-proxy into a free-standing library. This library could be managed and versioned separately from cql-proxy proper and could provide an API for the following functions:

• SCB creation from various inputs (URLs, paths, possibly the raw materials that make up an SCB, etc.)
• SCB interrogation
• SCB comparison

Doing so would give us a single place for this logic to live; customers can then use it in whatever way suits them.

It would also allow us to decouple gocql-astra from cql-proxy. At the moment gocql-astra imports the SCB handling fns from cql-proxy to deal with SCBs. Importing the (much smaller) lib being discussed here would provide a clearer picture of the actual dependencies and avoid the weird condition of gocql-astra depending upon the whole of cql-proxy (when it really doesn't).