Require callers of AcceptBlockHeader() to perform anti-dos checks

In order to prevent memory DoS, we must ensure that we don't accept a new
header into memory until we've performed anti-DoS checks, such as verifying
that the header is part of a sufficiently high work chain. This commit adds a
new argument to AcceptBlockHeader() so that we can ensure that all call-sites
which might cause a new header to be accepted into memory have to grapple with
the question of whether the header is safe to accept, or needs further
validation.

This patch also fixes two places where low-difficulty-headers could have been
processed without such validation (processing an unrequested block from the
network, and processing a compact block).

Credit to Niklas Gögge for noticing this issue, and thanks to Sjors Provoost
for test code.

test/functional/rpc_blockchain.py

@@ -452,8 +452,9 @@ class BlockchainTest(BitcoinTestFramework):
         # (Previously this was broken based on setting
         # `rpc/blockchain.cpp:latestblock` incorrectly.)
         #
-        b20hash = node.getblockhash(20)
-        b20 = node.getblock(b20hash)
+        fork_height = current_height - 100 # choose something vaguely near our tip
+        fork_hash = node.getblockhash(fork_height)
+        fork_block = node.getblock(fork_hash)
 
         def solve_and_send_block(prevhash, height, time):
             b = create_block(prevhash, create_coinbase(height), time)
@@ -461,10 +462,10 @@ class BlockchainTest(BitcoinTestFramework):
             peer.send_and_ping(msg_block(b))
             return b
 
-        b21f = solve_and_send_block(int(b20hash, 16), 21, b20['time'] + 1)
-        b22f = solve_and_send_block(b21f.sha256, 22, b21f.nTime + 1)
+        b1 = solve_and_send_block(int(fork_hash, 16), fork_height+1, fork_block['time'] + 1)
+        b2 = solve_and_send_block(b1.sha256, fork_height+2, b1.nTime + 1)
 
-        node.invalidateblock(b22f.hash)
+        node.invalidateblock(b2.hash)
 
         def assert_waitforheight(height, timeout=2):
             assert_equal(