Evaluate missing effects, and consider exposing txmeta effects

[ire-and-curses]

Currently, Horizon's effects support is incomplete. See e.g. #166. Another missing effect is fee support. These missing effects are needed by certain use cases and anecdotally have led to parallel ingestion projects such as stellar.expert, in order to get the necessary data.

We should evaluate what can be done to provide full effect support in Horizon.

Points to note:

• There may be performance considerations to orderbook-related effects.
• New ingestion may make this easier than it has been in the past.
• We should consider whether exposing txmeta events makes sense, and if so, whether that could be a viable replacement for the existing effects approach. The big advantage in this case is to retain a canonical event format that matches the underlying blockchain. How viable this is depends on the user-friendliness of that format, however.
• There may be other effects missing - the first step should be an audit of what we have.

[bartekn]

Related so adding to this issue:

We should also make sure that effects ingestion code rely more on txmeta. It works in the ideal world but doesn't take into account the bugs that happened in pubnet history. A good example is hash=f06c6a4. The ingestion code correctly creates account_debited and account_credited based on transaction and tx results but the meta is actually different (due to a account_merge bug from 2016).

It looks like ideally we should use io.Change . If Post.Balance>Pre.Balance then account_credited, else if Post.Balance<Pre.Balance then account_debited.