rest: fetch spent transaction outputs by blockhash

Today, it is possible to fetch a block's spent prevouts in order to
build an external index by using the `/rest/block/HASH.json` endpoint.
However, its performance is low due to JSON serialization overhead.

We can significantly optimize it by adding a new REST endpoint, using
a binary response format:

```
$ BLOCKHASH=00000000000000000002a7c4c1e48d76c5a37902165a270156b7a8d72728a054

$ ab -k -c 1 -n 100 http://localhost:8332/rest/block/$BLOCKHASH.json
Document Length:        13278152 bytes
Requests per second:    3.53 [#/sec] (mean)
Time per request:       283.569 [ms] (mean)

$ ab -k -c 1 -n 10000 http://localhost:8332/rest/spentoutputs/$BLOCKHASH.bin
Document Length:        195591 bytes
Requests per second:    254.47 [#/sec] (mean)
Time per request:       3.930 [ms] (mean)
```

Currently, this PR is being used and tested by Bindex:

 * https://github.com/romanz/bindex-rs

This PR would allow to improve the performance of external indexers
such as electrs, ElectrumX, Fulcrum and Blockbook:

 * https://github.com/romanz/electrs (also https://github.com/Blockstream/electrs and https://github.com/mempool/electrs)
 * https://github.com/spesmilo/electrumx
 * https://github.com/cculianu/Fulcrum
 * https://github.com/trezor/blockbook

test/functional/test_framework/messages.py

@@ -228,6 +228,11 @@ def ser_string_vector(l):
     return r
 
 
+def deser_block_spent_outputs(f):
+    nit = deser_compact_size(f)
+    return [deser_vector(f, CTxOut) for _ in range(nit)]
+
+
 def from_hex(obj, hex_string):
     """Deserialize from a hex string representation (e.g. from RPC)