Improve db-memory

[ineiti]

As it doesn't seem to be possible to reduce bbolt's memory usage, one should reduce the number of lookups:

• remove stateChangeStorage.calculateSize and all related size-handling of the db
• GetAllSkipchains and GetAllSkipchainIDs should not crawl through every block, but rather depend on a new table that holds all skipchains
• find the last places where new byzcoin-dbs are created
• use skipBlockShort for all methods, like SkipBlockDB.GetSingleBlockByIndex and others
• check if there are other places where the whole chain is crawled and all blocks are loaded into memory (db.Stats() or db.Status())
• eventually clean up the chain and drop unused blocks