best_containing operations (issue 603) (#740)

* add stub for Client.best_chain_containing_block_hash

* add fn blockchain::Backend::leaf_hashes

* fix typo

* sketch out Client.best_chain_containing_block_hash

* fix indent

* Blockchain.leaf_hashes -> Blockchain.leaves

* add unimplemented! stub impls for Blockchain.leaves

* start impl of Blockchain.leaves for in-memory client db

* Client.best_chain_containing...: check canonical first and make compile

* first rough attempt at maintaining leaf list in in-memory db

* fix tab indent

* add test best_chain_containing_single_block

* add failing test best_chain_containing_with_fork

* pub use client::blockchain; in test-client to prevent circular dep in client tests

* best_chain_containing_with_single_block: improve and test leaves

* far improve in-memory Backend::leaves impl

* test blockchain::Backend::leaves more thoroughly

* handle more edge cases in blockchain::Backend::leaves impl for in memory

* fix test best_chain_containing_with_fork (two distinct test blocks had same hash)

* make best_chain_containing_block_hash pass existing tests

* improve docstring for Blockchain::leaves

* Client.best_chain_containing: some cleanup. support max_block_number

* best_chain_containing: remove broken outcommented fast check for best = canonical

* remove blank line

* best_block_containing: return None if target_hash not found

* best_chain_containing: add unreachable! at end of function

* improve tests for best_chain_containing

* renames

* more elaborate test scenario for best_containing

* best_containing: fix restriction of search through maybe_max_number

* best_containing: tests for restriction of search

* get rid of unnecessary clones

* replace Client::new_in_mem by new_with_backend which is useful for testing backends

* add test_client::new_with_backend for testing different backend impls

* add test for in_mem::Backend::leaves

* remove unused imports

* in_mem test_leaves: simplify

* flesh out tests for in_mem leaves impl

* remove tests for leaves from client which are now covered in implementing module

* improve comment

* add Client.new_in_mem again

* unwrap in test_client::new_with_backend

* make test_client::BlockBuilderExt work not just with in-mem backend

* make test client ext not just work with in mem backend

* add failing Backend.leaves test for client-db

* update Cargo.lock

* replace KeccakHasher with Blake2Hasher

* refactor

address grumble https://github.com/paritytech/substrate/pull/740#discussion_r217822862

* refactor using NumberFor

address grumble https://github.com/paritytech/substrate/pull/740#discussion_r217823341

* add test that exposes possible problem

* update docstring for Client.best_containing

* extract test for Backend.leaves for reuse

* improve test blockchain_header_and_hash_return_blocks_from_canonical_chain_given_block_numbers

* extract test_blockchain_query_by_number_gets_canonical to easily test multiple impls

* remove whitespace

* remove todo

* Client.best_containing: pre-empt search loop when target in canonical

* best_containing: prevent race condition by holding import lock

* add todo

* extract leaf list update code into function

* add comment

* client-db: use in-memory-kvdb for tests

* use BTreeSet to store leaves for in-mem which is faster and simpler

* add docstring

* add comments and fix formatting

* add initial raw version of LeafSet

* remove Client::update_leaves which has been superceded by LeafSet

* use LeafSet in in-mem backend

* keccak -> blake2

* don't reexport codec traits in primitives

addresses https://github.com/paritytech/substrate/pull/740#discussion_r219538185

* fix rebase mistake

* improve LeafSet and use it in state-db

* correct Transfer nonces to fix ApplyExtinsicFailed(Stale)

* use given backend in canoncal test

* kill dead tree-route code in util

* fix warnings

* tests for leafset

* reorganizations in in_mem backend

* fix reorganization canon block logic

* DB commit and safe reversion on write error

* fix style nits