Storage format: Indexation direction

[d-r-q]

Currently qbit performs initial wal indexation in backwards order (from leaf to root). But this approach has several disadvantages:

• At the begin of an indexation, there is no information about db schema
• It worse matches (probable) patches storage (Storage format: full entity vs patch #9)
• It requires to store tombstones in index, to prevent insertion of deleted entitiies

The main advantage of this approach is that it doesn't require mutable forward trx graph index.
Also backward indexation may use immutable information from merge nodes about merged branches base.

On other hand current "backward" indexation, actually traverses a graph in backward direction and builts subgraph to index, and then traverses that graph in forward direction.

Consider switch to forward indexation direction. Especially, if #112 with append will be implemented.