Refactor mempoolIDs

[tzdybal]

mempoolIDs struct was pulled from Tendermint's mempool into node package. It's a bit confusing right now. Tendermint mempool (which is reused in Optimint right now) use IDs internally, but this information is derived from P2P information. Optimint decouples mempool operation and P2P networking.

We can change visibility of mempoolIDs in imported mempool package, or rename mempoolIDs to something more meaningful. Probably this can be also move out from node package, as it's kind of glue code, required only because Tendermint mempool is reused.