Long unconfirmed transaction chain doesn't get mined in regtest

Steps to reproduce (with 0.21.0.1):

```bash
elementsd -chain=elementsregtest -initialfreecoins=2100000000000000 -anyonecanspendaremine=1 -validatepegin=0 -datadir=/tmp/elements-test

alias cli='elements-cli -chain=elementsregtest -datadir=/tmp/elements-test'

cli createwallet default
cli rescanblockchain # pick up anyonecanspend

addr=$(cli getnewaddress)
for i in {1..26}; do cli sendtoaddress $addr 1; done

# the 26th transaction results in the following log message:
# [default] CommitTransaction(): Transaction cannot be broadcast immediately, too-long-mempool-chain, too many unconfirmed ancestors [limit: 25]
# (but `sendtoaddress` still replies successfully with its txid)

cli getmempoolinfo | jq .size
# prints 25 (the 26th transaction was ignored)

cli generatetoaddress 1 $addr

cli getmempoolinfo | jq .size
# still prints 25 -- all the transactions seem to be stuck in the mempool and don't get selected into mined blocks
```

#206 reported a similar issue (but possibly due to a different cause?).