No longer use internally the name slave DB for the mirror DB

[bschmalhofer]

One or more mirror databases can be set up to provide separate databases that can be used for reporting. This avoids a slowdown of OTOBO when reports, or statistics, are generated.
The config settings for the mirror database have the prefix Core::MirrorDB::. This is fine. But internally the name slave DB is still used. Most of the usage can be adapted in Kernel/System/DB.pm . One exception is the package variable Kernel::System::DB::UseSlaveDB which is used in OTOBO core and can be used in OTOBO packages.

I suggest to rename the internal variables and the package variable in OTOBO core. For the packages there are two options:

• still support the legacy name UseSlaveDB
• force a renaming when packages are adapted for OTOBO 11

[bschmalhofer]

Talked with @svenoe : renaming the global variable is fine

[bschmalhofer]

Merged the PR. Note that most of the commit were attached to #2430 because the changes were initially made in a branch for that issue.
Closing the issue.