[refactor] remove calling GetLowestVersion where possible

This change stops calling keeper.GetLowestActiveVersion() in all places except once (in manager.go). This expensive kvstore function is now only called a single time per block, instead of potentially multiple times per transaction and even per thornode API query.

Also, in an effort for uniformity, renamed keeper.Version() --> keeper.GetVersion() to match the similar function name in mgr.GetVersion()

Might be easiest to review this PR commit by commit.