reap max bytes from the mempool & check transaction size

See ADR 020: Limiting txs size inside a block docs/architecture/adr-020-block-size.md

Refs #2035

docs/architecture/adr-020-block-size.md

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+# ADR 020: Limiting txs size inside a block
+
+## Changelog
+
+13-08-2018: Initial Draft
+15-08-2018: Second version after Dev's comments
+28-08-2018: Third version after Ethan's comments
+30-08-2018: AminoOverheadForBlock => MaxAminoOverheadForBlock, added MaxAminoOverheadForTx
+
+## Context
+
+We currently use MaxTxs to reap txs from the mempool when proposing a block,
+but enforce MaxBytes when unmarshalling a block, so we could easily propose a
+block thats too large to be valid.
+
+We should just remove MaxTxs all together and stick with MaxBytes, and have a
+`mempool.ReapMaxBytes`.
+
+But we can't just reap BlockSize.MaxBytes, since MaxBytes is for the entire block,
+not for the txs inside the block. There's extra amino overhead + the actual
+headers on top of the actual transactions + evidence + last commit.
+
+## Proposed solution
+
+Therefore, we should
+
+1) Get rid of MaxTxs.
+2) Rename MaxTxsBytes to MaxBytes.
+
+When we need to ReapMaxBytes from the mempool, we calculate the upper bound as follows:
+
+```
+ExactLastCommitBytes = {number of validators currently enabled} * {MaxVoteBytes}
+MaxEvidenceBytesPerBlock = MaxBytes / 10
+ExactEvidenceBytes = cs.evpool.PendingEvidence(MaxEvidenceBytesPerBlock) * MaxEvidenceBytes
+
+mempool.ReapMaxBytes(MaxBytes - MaxAminoOverheadForBlock - ExactLastCommitBytes - ExactEvidenceBytes - MaxHeaderBytes)
+```
+
+where MaxVoteBytes, MaxEvidenceBytes, MaxHeaderBytes and MaxAminoOverheadForBlock
+are constants defined inside the `types` package:
+
+- MaxVoteBytes - 170 bytes
+- MaxEvidenceBytes - 364 bytes
+- MaxHeaderBytes - 476 bytes (~276 bytes hashes + 200 bytes - 50 UTF-8 encoded
+  symbols of chain ID 4 bytes each in the worst case + amino overhead)
+- MaxAminoOverheadForBlock - 4 bytes (assuming MaxHeaderBytes includes amino
+  overhead for encoding header, MaxVoteBytes - for encoding vote, etc.)
+
+NOTE while reaping the `max int` bytes in mempool, we should account that every
+transaction will take `len(tx)+aminoOverhead`, where aminoOverhead=1-4 bytes.
+Therefore, MaxAminoOverheadForTx should be added.
+
+We should write a test that fails if the underlying structs got changed, but
+MaxXXX stayed the same.
+
+## Status
+
+Proposed.
+
+## Consequences
+
+### Positive
+
+* one way to limit the size of a block
+* less variables to configure
+
+### Negative
+
+* constants that need to be adjusted if the underlying structs got changed
+
+### Neutral