Description
-----------
While implementing WL#12966 and WL#12968, decision was made to not clear
`PRIVILEGE_CHECKS_USER` and `REQUIRES_ROW_FORMAT` options values when performing
a `RESET SLAVE`. The state of such options is not kept if after a `RESET SLAVE`
the server is restarted.

Analysis
--------
Although the option values are kept in memory after a `RESET SLAVE`, the same
values are not persisted into the configured repository in order to be reloaded
into memory after a server restart.

Fix
---
1) Make `Relay_log_info` fields nullable.
2) Make `RESET SLAVE` clear all field values except for `PRIVILEGE_CHECKS_USER`
   and `REQUIRE_ROW_FORMAT`.
3) Persist the cleared record into the relay-log repositories.
4) Change boot logic so that the cleared record is used to instantiate
   `PRIVILEGE_CHECKS_USER` and `REQUIRE_ROW_FORMAT` and then relay log is
   initialized as before.

RB: 23376
Reviewed-by: Pedro Gomes <pedro.gomes@oracle.com>
Reviewed-by: Luís Soares <luis.soares@oracle.com>