MEDIUM: connection: replace idle conn lists by eb trees

MEDIUM: connection: replace idle conn lists by eb trees

The server idle/safe/available connection lists are replaced with ebmb-
trees. This is used to store backend connections, with the new field
connection hash as the key. The hash is a 8-bytes size field, used to
reflect specific connection parameters.

This is a preliminary work to be able to reuse connection with SNI,
explicit src/dst address or PROXY protocol.