Set InitMessage for txindex migration
This makes it so that when RPC calls are tried during the initial migration
(which takes ~ an hour on mainnet) they error with "-28 Upgrading txindex database"
instead of the misleading "-28 Verifying blocks...".
(why it's needed: only InitMessage is connected to SetRPCWarmupStatus)
Depends on D2924
build; try it out
On testnet I confirm that during migration, running bitcoin-cli getinfo gives:
error code: -28 error message: Upgrading txindex database
Reviewers: jasonbcox, Fabien, #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3115