MEDIUM: cache: Manage a subset of encodings in accept-encoding normalizer

MEDIUM: cache: Manage a subset of encodings in accept-encoding normalizer

The accept-encoding normalizer now explicitely manages a subset of
encodings which will all have their own bit in the encoding bitmap
stored in the cache entry. This way two requests with the same primary
key will be served the same cache entry if they both explicitely accept
the stored response's encoding, even if their respective secondary keys
are not the same and do not match the stored response's one.
The actual hash of the accept-encoding will still be used if the
response's encoding is unmanaged.
The encoding matching and the encoding weight parsing are done for every
subpart of the accept-encoding values, and a bitmap of accepted
encodings is built for every request. It is then tested upon any stored
response that has the same primary key until one with an accepted
encoding is found.
The specific "identity" and "*" accept-encoding values are managed too.
When storing a response in the key, we also parse the content-encoding
header in order to only set the response's corresponding encoding's bit
in its cache_entry encoding bitmap.

This patch fixes GitHub issue #988.
It does not need to be backported.