* Avoid creation of map with all archive contents; instead parse packages
while walking though the archive
* Avoid instantiation of std::string in come cases (using std::string_view)
* Reuse libarchive's archive entry when walking though archive
* Use visitor-based database parser in all places to avoid intermediate big
array with all package objects
Especially when enabling files DBs it is quite problematic to store all
package objects of a database within one big array. This change avoids the
array and instead adds the packages directly. The disadvantage is that
clearing the database isn't as simple anymore.
* Only lock the config for writing the reloading the config file
* Make sure all write operations to the database acquire an "update mutex"
to ensure only one write operation happens at a time
* Do *not* acquire any additional locks when reading from a database as it
should be safe to do so even when a write operation happens because
* LMDB read and write transactions can happen at the same time
* The package cache has its own mutex anyways
* Write ops to the package cache try to lock the "update mutex" to
prevent writing "old" data to the cache during updates
* Make "lastUpdate" atomic to avoid locking the config when accessing it
* Do HTTP head request first when loading database from mirror to avoid
downloading the full database all the time
* Use the last modification date of the local database file because with
the persistent storage even local database reloads became a bit expensive
Martchus