pkg/index: change deletes cache

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