Merge PR #2334: ADR for global message counter

docs/architecture/decision-records/adr-001-message-counter.md

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+# ADR 001: Global Message Counter
+
+## Context
+
+There is a desire for modules to have a concept of orderings between messages.
+
+One such example is in staking, we currently use an "intra bond tx counter" and
+bond height.
+The purpose these two serve is to providing an ordering for validators with equal stake,
+for usage in the power-ranking of validators.
+We can't use address here, as that would create a bad incentive to grind
+addresses that optimized the sort function, which lowers the private key's
+security.
+Instead we order by whose transaction appeared first, as tracked by bondHeight
+and intra bond tx counter. 
+
+This logic however should not be unique to staking.
+It is very conceivable that many modules in the future will want to be able to
+know the ordering of messages / objects after they were initially created.
+
+## Decision
+
+Create a global message counter field of type int64.
+Note that with int64's, there is no fear of overflow under normal use,
+as it is only getting incremented by one,
+and thus has a space of 9 quintillion values to go through.
+
+This counter must be persisted in state, but can just be read and written on 
+begin/end block respectively.
+This field will get incremented upon every DeliverTx,
+regardless if the transaction succeeds or not. 
+It must also be incremented within the check state for CheckTx.
+The global message ordering field should be set within the context
+so that modules can access it.
+
+## Corollary - Intra block ordering
+In the event that there is desire to just have an intra block msg counter,
+this can easily be derived from the global message counter.
+Simply subtract current counter from first global message counter in the block.
+Thus the relevant module could easily implement this.
+
+## Status
+Proposed
+
+## Consequences
+
+### Positive
+* Moves message ordering out of the set of things staking must keep track of
+* Abstracts the logic well so other modules can use it
+
+### Negative
+* Another thing to implement prelaunch. (Though this should be easy to implement)
+
+### Neutral