* Restrict access to Bank's HardForks
Callers could previously obtain a a lock to read/write HardForks from
any Bank. This would allow any caller to modify, and creates the
opportunity for inconsistent handling of what is considered a valid hard
fork (ie too old).
This PR adds a function to Bank so consistent sanity checks can be
applied; the caller will already have a Bank as that is where they would
have obtained the HardForks from in the first place. Additionally,
change the getter to return a copy of HardForks (simple Vec).
* Allow hard fork at bank slot if bank is not yet frozen