Do not send more than one request per peer at a time

[tomaka]

This PR introduces a "request lock" system to the NetworkService of the light client.
When a "request lock" is held, no other request lock with the same PeerId can be created. A "request lock" can then be used to start an actual request.

This causes the sync_service (and all parts of the node) to only start one request per peer at a time.
This effectively implements an existing requirement of the Polkadot networking protocol that isn't enforced by Substrate.

There are some caveats and further PRs will further refine this system:

• The storage subscriptions start many storage requests, while only one could be enough.
• The JSON-RPC server should maybe batch the calls to the legacy API that must be answered with a request.
• In order for this to make sense with the new API, Allow querying and returning multiple items at once in chainHead_unstable_storage paritytech/json-rpc-interface-spec#51 should be implemented.

[tomaka]

Just tried rebasing this PR, and I think that it's easier to restart it from scratch.