* Alerting: Increase size of kvstore value type for MySQL to LONGTEXT
alertmanager uses the kvstore to persist its notification log and the current
column limit for MySQL (16.7mb) puts the maximum entries at a level that is
potentially achievable for heavy alerting users (~40-80k entries).
In comparison, the current type for PSQL (TEXT) is effectively unlimited and
I believe SQLIte defaults to 2gb which is also plenty of leeway.
* add key/value store service
* don't export kvStoreSQL, consumers should interact with KVStore & NamespacedKVStore
* add del method, avoid ErrNotFound (#38627)
* switch value column to medium text
Co-authored-by: Alexander Emelin <frvzmb@gmail.com>
