feat(db): add LastFetchedAt in fetchmeta

[MaineK00n]

What did you implement:

Add the time when the fetch is completed to fetchmeta.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

RDB

$ go-cve-dictionary fetch nvd
$ sqlite3 cve.sqlite3  
sqlite> SELECT last_fetched_at FROM fetch_meta;
2021-12-26 08:45:48.769925148+09:00

$ go-cve-dictionary fetch nvd
$ sqlite3 cve.sqlite3  
sqlite> SELECT last_fetched_at FROM fetch_meta;
2021-12-26 08:50:40.68298214+09:00

Redis

$ redis-cli -p 6379
127.0.0.1:6379> HGET CVE#FETCHMETA LastFetchedAt
(nil)

$ go-cve-dictionary fetch nvd --dbtype redis --dbpath "redis://127.0.0.1:6379/0"
$ redis-cli -p 6379
127.0.0.1:6379> HGET CVE#FETCHMETA LastFetchedAt
"2021-12-26T08:52:51.973265483+09:00"

$ go-cve-dictionary fetch nvd --dbtype redis --dbpath "redis://127.0.0.1:6379/0"
$ redis-cli -p 6379
127.0.0.1:6379> HGET CVE#FETCHMETA LastFetchedAt
"2021-12-26T08:54:48.1066968+09:00"

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES

Reference