Remove 'ephemeral' database implementation

Remove 'ephemeral' database implementation

The 'ephemeral' database implementation was used to provide a
lightweight database implemenation that doesn't cache results, and the
only place where it was really use is "samples" because delv is
overriding this to use "rbtdb" instead. Otherwise it was completely
unused.

 * The 'ephemeral' cache DB (ecdb) implementation.  An ecdb just provides
 * temporary storage for ongoing name resolution with the common DB interfaces.
 * It actually doesn't cache anything.  The implementation expects any stored
 * data is released within a short period, and does not care about the
 * scalability in terms of the number of nodes.