add min persistent committee period resitriction on exits

specs/core/0_beacon-chain.md

@@ -231,6 +231,7 @@ Code snippets appearing in `this style` are to be interpreted as Python code.
 | `EPOCHS_PER_ETH1_VOTING_PERIOD` | `2**4` (= 16) | epochs | ~1.7 hours |
 | `SLOTS_PER_HISTORICAL_ROOT` | `2**13` (= 8,192) | slots | ~13 hours |
 | `MIN_VALIDATOR_WITHDRAWABILITY_DELAY` | `2**8` (= 256) | epochs | ~27 hours |
+| `PERSISTENT_COMMITTEE_PERIOD` | `2**11` (= 2,048)  | epochs | 9 days  |
 
 ### State list lengths
 
@@ -2463,6 +2464,8 @@ def process_exit(state: BeaconState, exit: VoluntaryExit) -> None:
     assert validator.exit_epoch > get_delayed_activation_exit_epoch(get_current_epoch(state))
     # Exits must specify an epoch when they become valid; they are not valid before then
     assert get_current_epoch(state) >= exit.epoch
+    # Must have been in the validator set long enough
+    assert get_current_epoch(state) - validator.activation_epoch >= PERSISTENT_COMMITTEE_PERIOD
     # Verify signature
     assert bls_verify(
         pubkey=validator.pubkey,