Consistently use ParseHashV to validate hash inputs in rpc

ParseHashV validates the length and encoding of the string and throws
an informative RPC error on failure, which is as good or better than
these alternative calls.

Note I switched ParseHashV to check string length first, because
IsHex tests that the length is even, and an error like:
"must be of length 64 (not 63, for X)" is much more informative than
"must be hexadecimal string (not X)"

test/functional/p2p_compactblocks.py

@@ -287,7 +287,7 @@ class CompactBlocksTest(BitcoinTestFramework):
         block_hash = int(node.generate(1)[0], 16)
 
         # Store the raw block in our internal format.
-        block = FromHex(CBlock(), node.getblock("%02x" % block_hash, False))
+        block = FromHex(CBlock(), node.getblock("%064x" % block_hash, False))
         for tx in block.vtx:
             tx.calc_sha256()
         block.rehash()