rename CDDL timelock variable names

For consistency, we are going to uniformly use "invalid_before" and
"invalid_hereafter" to denote the half open validity intervals
[invalid_before, invalid_hereafter).

shelley-ma/shelley-ma-test/cddl-files/shelley-ma.cddl

@@ -230,8 +230,12 @@ native_script =
  // script_all
  // script_any
  // script_n_of_k
- // timelock_start
- // timelock_expiry
+ // invalid_before
+ ; Timelock validity intervals are half-open intervals [a, b).
+ ; This field specifies the left (included) endpoint a.
+ // invalid_hereafter
+ ; Timelock validity intervals are half-open intervals [a, b).
+ ; This field specifies the right (excluded) endpoint b.
  ]
 
 script_pubkey = (0, addr_keyhash)