This allows a `"if_block_not_equal": "hash"` parameter to be given to
`get_n_service_nodes` which, if the given value matches the current
top block hash, skips building and returning a reply.

This is primarily aimed at lokinet which polls fairly frequenty for an
update but where the vast majority of those polls contain no new
information.

It also removes the get_all_service_nodes_public_keys RPC request
completely as lokinet was the only consumer of it (particularly unlikely
that anyone else was using this because it was returning the keys
base32z-encoded).

With the previous commit we no longer need this conversion (and lokinet
is perfectly happy just getting hex encoded values instead).

Renamed:
  - `if_block_not_equal` -> `poll_block_hash`
  - `gave_if_not_equal` -> `polling_mode`

Added lokidocs documentation for the new and existing get_n_service_node
fields, and adds magic codes to disable the internal variables.