spec: move to final location (#1576)

docs/spec/reactors/mempool/functionality.md

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+# Mempool Functionality
+
+The mempool maintains a list of potentially valid transactions,
+both to broadcast to other nodes, as well as to provide to the
+consensus reactor when it is selected as the block proposer.
+
+There are two sides to the mempool state:
+
+* External: get, check, and broadcast new transactions
+* Internal: return valid transaction, update list after block commit
+
+
+## External functionality
+
+External functionality is exposed via network interfaces
+to potentially untrusted actors.
+
+* CheckTx - triggered via RPC or P2P
+* Broadcast - gossip messages after a successful check
+
+## Internal functionality
+
+Internal functionality is exposed via method calls to other
+code compiled into the tendermint binary.
+
+* Reap - get tx to propose in next block
+* Update - remove tx that were included in last block
+* ABCI.CheckTx - call ABCI app to validate the tx
+
+What does it provide the consensus reactor?
+What guarantees does it need from the ABCI app?
+(talk about interleaving processes in concurrency)
+
+## Optimizations
+
+Talk about the LRU cache to make sure we don't process any
+tx that we have seen before