refs #346

We'll soon have some members we don't want to swap, which would happen
if we were using std::swap

We can infer the next wake up time since this was only used when all
the queues were empty.
This reduces the need for a global lock when computing the next wake up
time
refs #346

It is accessed with the mutex held almost all the times, except when
setting it.
refs #346

Fix #368

Fix #338

This will simplify the triggers when we start to use that column to
expose a number of seen media for media groups

Fix #235

This allows the user to know what's contained in a playlist without
listing the media, and will allow the application to filter playlist by
their content type
Refs #235

Fix #321

This matches what's done for the removal

It will update more than just the media count in the next commit

Fix #268

This being a valuable performance boost, especially with long strings,
which is usually the case for the SQL request cache

This saves a temporary std::string construction/destruction each time
the hook is invoked, which is at least once per modification request,
and more when triggers are involved

This will allow heterogeneous lookups in std::unordered_map in order to
remove the need of creating a temporary string each time we perform a
lookup with a const char*

This mostly fixes cppcheck warnings about value not being read