Simplify chain event Called API

[dirkmc]

A couple of simplifications to the chain event Called API:

1. The match function returns three parameters:

• match once
  if true: if there are multiple listeners registered to listen for a particular message, only callback the first one
• matched
  true if there was a match
• error

In practice, everywhere in the code that Called is used, match once is always false. I can't think of a reason why you would only want the first registered listener to match a method to get a callback and ignore the other listeners, so this PR removes the match once return value.

2. messageEvents has a map map[triggerID][]MsgMatchFunc
   But there is only ever one element in the array. So just map directly to a MsgMatchFunc instead of an array:
   map[triggerID]MsgMatchFunc

[magik6k]

Needed to dig a bit deeper to convince myself that this is not breaking anything we could have forgotten about

LGTM

[magik6k]

If anyone is reviewing this PR later:

https://github.com/filecoin-project/lotus/pull/2246/files#diff-2c1843994ac72e7b0c9c419e81d77df0c3573f9858f2474632411b8bec005904R505-R507 introduced matchOne which made this break conditional, but really it just shouldn't be there.