datastore: SnapshotBundle schema

[battlmonstr]

Previously SnapshotBundle contained a fixed number of predefined entities.
Now SnapshotBundle is dynamic and based on a snapshots::Schema::RepositoryDef. The actual schema is defined in db::blocks::make_blocks_repository_schema(). snapshots::Schema follows a "fluent config builder" pattern. Currently rec_split_index_defs_ values are trivial there, but in the future we will configure index builders there.

snapshots::Schema::RepositoryDef is a part of datastore::Schema. It is configured by db::DataStore::make_schema.
Currently datastore::Schema just wraps snapshots::Schema, but in the future it will also contain DHIIs.

For the DAL client code a db::blocks::BundleDataRef wrapper is provided for convenient access into the block repository bundles (much like db::DataStoreRef provides a convenient access to the block repository).

Related refactorings:

• separate bundle and bundle paths. SnapshotBundlePaths is useful for file manipulation without opening snapshots.
• always open SnapshotBundle on creation (RAII)
• open repository by default. It is not opened only in case of CAPI where bundles are mmap-ed and provided externally.
• move db/transactions subfolder into db/blocks. db/blocks now contains everything related to "block snapshots". db/state will contain "state snapshots" and DHII.
• rename snapshot_repository variables to just repository