Db name of the database is stored in two fields of DatabaseName. 

• _dbName,  it holds the db name
• _tenantDbName, it holds the tenant id and db name in one string when a tenant id is assigned.

In this ticket, we will redesign the internal data struct to have the db name only be stored in one place, such as _name. This field will hold a string with

•  <tenant id>_<db name>, if tenant id is assigned.
•  <db name>, otherwise.

Also, add a field to store the start index of db name and change the implementations of related functions, for example
StringData dbName() {
return StringData(_name.c_str() + _dbIndex, _tenantDbName.size());
}

As a part of this ticket, also remove createSystemTenantDbName - we are not currently using it anywhere and it breaks convention as it takes in a string without a tenantId or a separate tenantId param.