Extract warp sync strategy from ChainSync (#2467)

Extract `WarpSync` (and `StateSync` as part of warp sync) from
`ChainSync` as independent syncing strategy called by `SyncingEngine`.
Introduce `SyncingStrategy` enum as a proxy between `SyncingEngine` and
specific syncing strategies.

## Limitations
Gap sync is kept in `ChainSync` for now because it shares the same set
of peers as block syncing implementation in `ChainSync`. Extraction of a
common context responsible for peer management in syncing strategies
able to run in parallel is planned for a follow-up PR.

## Further improvements
A possibility of conversion of `SyncingStartegy` into a trait should be
evaluated. The main stopper for this is that different strategies need
to communicate different actions to `SyncingEngine` and respond to
different events / provide different APIs (e.g., requesting
justifications is only possible via `ChainSync` and not through
`WarpSync`; `SendWarpProofRequest` action is only relevant to
`WarpSync`, etc.)

---------

Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>

substrate/client/network/sync/Cargo.toml

@@ -49,7 +49,7 @@ sp-consensus-grandpa = { path = "../../../primitives/consensus/grandpa" }
 sp-runtime = { path = "../../../primitives/runtime" }
 
 [dev-dependencies]
-tokio = { version = "1.22.0", features = ["macros"] }
+mockall = "0.11.3"
 quickcheck = { version = "1.0.3", default-features = false }
 sc-block-builder = { path = "../../block-builder" }
 sp-test-primitives = { path = "../../../primitives/test-primitives" }