Instead of calling `pushKV(hash, info)`, which incurs in duplicate key checks, call `__pushKV` which (currently) doesn't.

Improves RPC `getrawmempool` and REST `/rest/mempool/contents.json`.

Fixes #14765.

git ch master
ninja bitcoind bitcoin-cli
bitcoind -rest -connect=0     # -connect=0 ensures we do not download any new transactions while testing
bitcoin-cli invalidateblock 000000000000000000e311a3b4f53085a427a22dcc8c790b266d9f882c795564  # some block that is not recent
bitcoin-cli getmempoolinfo    # confirm you have at least thousands of txs in the mempool
for X in {1..100} ; do { time curl localhost:8332/rest/mempool/contents.json ; } 2>> results-without-patch ; done

git ch <this-patch>
ninja bitcoind bitcoin-cli
bitcoind -rest -connect=0     # -connect=0 ensures we do not download any new transactions while testing
bitcoin-cli getmempoolinfo    # confirm you have the same number of transactions in the mempool as the previous test
for X in {1..100} ; do { time curl localhost:8332/rest/mempool/contents.json ; } 2>> results-with-patch ; done

cat results-without-patch | grep real | sed 's/real[ \t]//g' | sed 's/[ms]//g' | awk '{total+=$1; count+=1} END {print total/count}'
cat results-with-patch | grep real | sed 's/real[ \t]//g' | sed 's/[ms]//g' | awk '{total+=$1; count+=1} END {print total/count}'