Unpark the checkpointed blocks

Summary:
Now that avalanche is enabled by default, the node might park blocks pending
for polling after an upgrade. We can just unpark the checkpointed blocks upon
startup instead.

Test Plan:

Start bitcoind on mainnet, let it run out of IBD. Park a block shortly before the most recent checkpoint:

src/bitcoin-cli parkblock 00000000000000000036e1c7ca4d2c0d3de84fc5ba2a720a226eb14ab612ec0f

Check the result with the getchaintips RPC.

Stop and restart bitcoind, verify that the entire chain is unparked with getchaintips.
Look for the log message Unparking checkpointed block 00000000000000000b360176b8456de45b662fce6d557c6238dec17362d197f3 ... (this is the hash of the most recent checkpoint).

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13907