Implement validation logic for indexing zomes

[pospi]

There is some base Holochain stuff that needs validation in addition to the callbacks needing to be implemented for #177 #178 #145 #310. Please add to the list below as these constraints become clearer.

hdk_time_indexing

• Any links of a LinkTypes::TimeIndex can only be created between EntryHash and AgentPubKey nodes.
• In addition, AgentPubKey nodes may only occupy the leafmost nodes in a time index tree

hdk_semantic_indexing

• …

Agent zome

• LinkTypes::MyAgent links are only valid from an AgentPubKey to an EntryHash:
  • identifying the hash of an AgentAddress
  • where the referenced Entry of that AgentAddress is a hc_zome_rea_agent_storage::EntryStorage and EntryStorage.entry.agent_type == "Person"
• There can only be 1 LinkTypes::MyAgent link per Agent.