Remove Certificate Usages

- Modify text to no longer depend on Certifiocates stored in the
  overlay.
- Keep the Certificates usages in the document because self-signed
  certificates require to be stored under the CERTIFICATE_BY_USER usage,
  to check that a name is not alreday taken.
- Kept CERTIFICATE_BY_NODE even if it not used at all this -base.
- Add text saying that when using self-certificate, a node MUST store
  the certificate, as this is no longer needed for non-self-certificate
  based overlays.
- Changed peer to node when needed.
comment-58

- Fix as suggested.
comment-67

- Added reference toi ietf-p2psip-self-tuning.
Merge pull request #168 from petithug/remove-cert-usage

Remove Certificate Usages
Merge pull request #105 from petithug/comment-67

comment-67
clean up ref to not be normative
Merge pull request #104 from petithug/comment-58

comment-58
Merge branch 'master' of github.com:fluffy/p2psip-base-master
clean up lifetime text
update old to diff agains
Issue 143
Merge pull request #169 from fluffy/issue143

Issue 143
update date
move diff basis to -23 version
update date
moved to version 25 of draft