Transient stores

[ebuchman]

Sometimes we need a place to persist things between transactions that can be wiped on commit.

For instance in staking, we keep the TendermintUpdates around in the store, but they are cleared before commit.

We should consider how to introduce transient stores that can be used to make this clearer

See #1013

[mossid]

Possible implementations:

1. Mount transient substore in cacherootmultistore and make them not processed while .Write() (from staking TendermintUpdate Store should be in a transient store #1013)
2. Make separate multistore that only contains transient stores and make them accessible by Context.TransientStore as opposed to Context.KVStore

[mossid]

Should transient stores be a part of merkle tree? Since they are cleaned at the commit of each blocks, I don't think we have to make them as iavlStores.