Summary:
This diff makes two changes to improve logging for incremental catalog cache
refresh

(1) added DEBUG in the log text to make its purpose clearer
(2) changed `messages` to `invalidation messages` in the log to improve readability

I tried to also replace YBC_LOG_INFO with elog but that caused some unit tests to fail
because they have `SET client_min_messages TO log;`. One difference between
`YBC_LOG_INFO` and elog is that in case of `YBC_LOG_INFO` the log only goes into the
server log but not to the client.

Because incremental catalog cache refresh is a new feature, these `YBC_LOG_INFO` logging
are used to assist debugging. Once the feature is rolled out and field tested and we are
more confident that it is stable enough, I plan to make them DEBUG log and then replace
`YBC_LOG_INFO` with `elog`.
Jira: DB-16921

Test Plan:
./yb_build.sh release --cxx-test pg_catalog_version-test
Look at the new test logs. E.g.:

```
[ts-2] I0926 18:20:27.510396 1845785 pg_yb_utils.c:3256] YBCommitTransactionContainingDDL: got 26 invalidation messages, local catalog version 2

```

```
[ts-1] I0926 18:20:32.236284 1845757 pg_yb_utils.c:3114] DEBUG: pg null=0, nmsgs=4

```

Reviewers: jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D46997