Remove duplicate specification of features

Remove duplicate specification of features

Features for a given context are duplicated throughout the features
module. Use a macro for defining a Context and the applicable features
such that features only need to be defined for a Context in one place.
The Context provides bitmasks for selecting known and unknown feature
flags.

BOLT 1 and BOLT 9 refer to features as "known" if a peer understands
them. They also use the term "supported" to mean either optional or
required.

Update the features module to use similar terminology.
- Define contexts in terms of required and optional features rather than
  just supported features
- Define known features as those that are optional or required
- Rename supported() constructor to known()

For completeness, clear_optional_bit for each feature is now called
clear_bits and clears both optional and required bits.