Remove inFlightEcho entry on ECHO_REQ failure

[grom358]

In Gossiper, echoHandler only implements onResponse. RequestCallback.onFailure has a default no-op, so when the ECHO_REQ times out or the remote node returns an error, inflightEcho.remove(addr) is never called. The stale entry persists. Any subsequent markAlive(addr, localState) call — where localState is the same in-place-mutated object already in inflightEcho — sees localState.equals(prevState) = true (identity equality, same reference) and skips indefinitely. In a temporary-partition scenario (node briefly unreachable, echo times out, node recovers with the same generation), the node can get stuck permanently dead: the failure detector sees it as alive and keeps triggering markAlive, but every invocation is suppressed by the stale entry. The stale entry is only cleared by removeEndpoint() (explicit removal) or silentlyMarkDead() via markDead() (failure detector conviction) — neither of which fires if the failure detector is reporting the node as healthy.

Fix: override onFailure in echoHandler to call inflightEcho.remove(addr).