bound mempool memory usage (#3248)

* bound mempool memory usage

Closes #3079

* rename SizeBytes to TxsTotalBytes

and other small fixes after Zarko's review

* rename MaxBytes to MaxTxsTotalBytes

* make ErrMempoolIsFull more informative

* expose mempool's txs_total_bytes via RPC

* test full response

* fixes after Ethan's review

* config: rename mempool.size to mempool.max_txs

https://github.com/tendermint/tendermint/pull/3248#discussion_r254034004

* test more cases

https://github.com/tendermint/tendermint/pull/3248#discussion_r254036532

* simplify test

* Revert "config: rename mempool.size to mempool.max_txs"

This reverts commit 39bfa36961.

* rename count back to n_txs

to make a change non-breaking

* rename max_txs_total_bytes to max_txs_bytes

* format code

* fix TestWALPeriodicSync

The test was sometimes failing due to processFlushTicks being called too
early. The solution is to call wal#Start later in the test.

* Apply suggestions from code review

docs/tendermint-core/configuration.md

@@ -183,10 +183,15 @@ recheck = true
 broadcast = true
 wal_dir = ""
 
-# size of the mempool
+# Maximum number of transactions in the mempool
 size = 5000
 
-# size of the cache (used to filter transactions we saw earlier)
+# Limit the total size of all txs in the mempool.
+# This only accounts for raw transactions (e.g. given 1MB transactions and
+# max_txs_bytes=5MB, mempool will only accept 5 transactions).
+max_txs_bytes = 1073741824
+
+# Size of the cache (used to filter transactions we saw earlier) in transactions
 cache_size = 10000
 
 ##### consensus configuration options #####