pkg/index: change deletes cache structure and meaning

1) The deletes cache now simply reflects all the keyDeleted index entries,
instead of trying to always hold the computed deletion state of the
blobs.

2) The IsDeleted and DeletedAt methods have been changed accordingly.
Moreover, while they previously were only taking into account the most
recent delete claim that affected the input blob, they now take
into account all delete claims that concern said blob.
In other words, a blob is considered "active" only if all of its past
deleters are deleted too (or it does not have any deleter).

http://camlistore.org/issue/191

Change-Id: I1b276dac0b3c6a3fb3beaf96828a45a85d27e4c6
pkg/index: update the deletes cache when receiving a delete claim

http://camlistore.org/issue/191

Change-Id: I49da2ef4e43675fba6a80db29ba96a473c159403
Merge "pkg/index: change deletes cache structure and meaning"
Merge "pkg/index: update the deletes cache when receiving a delete claim"
Speed up in-memory permanode attribute search.

Finishes Corpus.AppendPermanodeAttrValues and uses it in
PermanodeConstraint.

Change-Id: I031789e0974add24886cc0060e52ba538535c4e1