[tablet] Split health_check_interval into two flags

[brirams]

Description

The health_check_interval flag controls:

• how often we send healthchecks to vtgates via the healthcheck stream
• how often the replication manager checks the health of its replication stream

we'd like to change the first without affecting the other so this PR:

• introduces the replication_health_check_interval for the latter
• defaults the resolved config value to the same value as the passed in health_check_interval if the new flag is not provided
• passed the new flag value into the TabletManager for use in constructing its replManager

Checklist

• Tests were added or are not required
• Documentation was added or is not required