Routing backlog

[ariard]

Opening this while reviewing #646 to track potential follow-ups.

• Move macros add_entry/add_entries_to_cheapest_to_target_node as utility functions using a RouterState wrapper to pass elements across calls.
• Upper dound payment path based on the bottleneck's htlc_max_value_in_flight, see https://github.com/rust-bitcoin/rust-lightning/pull/646#discussion_r506265712
• Sanitize channel_update reception with crappy values. Maybe add an event to allow custom sanitization, see https://github.com/rust-bitcoin/rust-lightning/pull/646#discussion_r514454426
• Verify naming nomenclature is consistent, see https://github.com/rust-bitcoin/rust-lightning/pull/646#discussion_r504774733 and https://github.com/rust-bitcoin/rust-lightning/pull/646#discussion_r504718057
• Move channel features sanitization at gossips reception instead of checking them in get_route, see https://github.com/rust-bitcoin/rust-lightning/pull/646#discussion_r514476009
• Add support for non-MPP receiving node, see https://github.com/rust-bitcoin/rust-lightning/pull/646#pullrequestreview-549629213
• Add a max "reasonable" fee, see https://github.com/rust-bitcoin/rust-lightning/pull/646#pullrequestreview-549629213