authorities_at cache update

[svyatonik]

closes #765

Besides non-instant finality support, there's also:

• several list-based caches can now share single db column (we'll need cache ":code" on light clients for sure);
• extracted cache-related source to the separate folder - there's too much code (especially tests) for single file.

The cache itself will behave optimal (without any db-requests), when:

• either we work with instant-finality consensus;
• or there are no forks in post-finalized blocks.

Otherwise, some additional lookups are performed when block is inserted/finalized or value is queried from the cache. Cache supports pruning entries for old finalized blocks. Cache destroys 'abandoned' unfinalized forks (i.e. forks that have forked before best finalized block).