lnutil: cache validate_features() results

channel_db.load_data() is slow, slowing down startup time (when trampoline is disabled).
util.list_enabled_bits() is one of the main contributors to the slowness, called by validate_features().

One could argue that we could even simply *not* call validate_features for gossip messages as part of load_data,
as they have already been validated before storing them in the db. However re-validating them there is a good
clean-up/sanity check IMO. Note that what is considered "valid" can change over time, so just because validate_features
passed when we originally received and stored a gossip message, it might no longer be valid a year later if the bolts change.

This caching decreases the time needed for load_data on two different machines / gossip dbs as below:
47 sec -> 10 sec
18 sec -> 6 sec

If instead of caching, I just rm the validate_features() calls, the benchmarks are almost identical, within noise.
That is, the cache looks really effective.
(the rest of the slowness is mostly due to lnmsg.decode_msg)

```
>>> lnutil.validate_features.cache_info()
CacheInfo(hits=172674, misses=287, maxsize=1000, currsize=277)
```

-----

We could alternatively directly cache util.list_enabled_bits (instead of validate_features).
That would be a bit slower and might end up using a lot more memory in some cases I think, but maybe conceptually would be cleaner.
Also note that if validate_features() raises an exception, that is not cached.

electrum/lnutil.py

@@ -7,6 +7,7 @@ from collections import defaultdict
 from typing import NamedTuple, List, Tuple, Mapping, Optional, TYPE_CHECKING, Union, Dict, Set, Sequence
 import sys
 import time
+from functools import lru_cache
 
 import electrum_ecc as ecc
 from electrum_ecc import CURVE_ORDER, ecdsa_sig64_from_der_sig
@@ -1801,6 +1802,7 @@ if hasattr(sys, "get_int_max_str_digits"):
     assert sys.get_int_max_str_digits() >= 4300, f"sys.get_int_max_str_digits() too low: {sys.get_int_max_str_digits()}"
 
 
+@lru_cache(maxsize=1000)  # massive speedup for the hot path of channel_db.load_data()
 def validate_features(features: int) -> LnFeatures:
     """Raises IncompatibleOrInsaneFeatures if
     - a mandatory feature is listed that we don't recognize, or