Access through a broadcastTransaction method.
Add a wait_callback flag to turn off race protection when wallet
already track its tx being in mempool
Standardise highfee, absurdfee variable name to max_tx_fee
We drop the P2P check in BroadcastTransaction as g_connman is only
called by RPCs and the wallet scheduler, both of which are initialized
after g_connman is assigned and stopped before g_connman is reset.
Depends on D6272
Partial backport of Core PR15713