Initial support for per-message TTLs (#6272)

This is an incomplete but somewhat-working implementations of
per-message TTLs.

Notes:
- Adds `AllowMsgTTL` to stream config;
- Parses the `Nats-TTL` message header as a parsable duration, or if
not, second precision;
- Bumps the `index.db` magic version so that we can add a new field for
tracking how many TTL'd messages are in each message block (means that
`index.db` needs to be rebuilt if downgrading);
- Adds a new `thw.db` to store the timed hash wheel state, and tries to
rebuild it from a linear scan if it is missing or out-of-date.

Future work:
- Memory store support;
- Expires "never" including not aging out with MaxAge;
- Ability to update a TTL for a message;
- Decide what we want to do for enabling/disabling TTL support;
- Decide how mirroring/sourcing streams will handle TTL headers.

Signed-off-by: Neil Twigg <neil@nats.io>

server/errors.json

@@ -1628,5 +1628,15 @@
     "help": "",
     "url": "",
     "deprecates": ""
+  },
+  {
+    "constant": "JSMessageTTLInvalidErr",
+    "code": 400,
+    "error_code": 10165,
+    "description": "invalid per-message TTL",
+    "comment": "",
+    "help": "",
+    "url": "",
+    "deprecates": ""
   }
 ]