p2p/discover: node database followup issues

[fjl]

The node database implemented in #793 has some issues that need to be resolved:

• The database manages to pick up the local node (Table.self) under some conditions.
  This should not happen. If the expiration (or the seed query) finds a node with the local ID,
  it should drop it.
• There needs to be a way to disable the seeding process from the command line.

[obscuren]

@fjl @karalabe update or close please

[karalabe]

I'll try and sort these out today.