stack overflow risk in on_prepare fast forward recursion

[Sertug17]

In src/consensus/role_follower.rs:230, on_prepare uses synchronous
recursion to fast forward through chained prepares:

    return self.on_prepare(prepare.0, prepare.1, prepare.2, prepare.3);

This is acknowledged in the code at line 227 with a FIXME comment.
Sync recursion in Rust uses the real call stack. With enough chained
prepares (e.g. after being offline for many slots), each call adds a
stack frame until the ~2MB stack overflows and the node crashes.

Suggested fix: convert the recursive call to an iterative loop by
wrapping the function body in loop { ... break; } where the fast
forward path reassigns local variables and continues instead of
recursing.