SyncConfiguration - compactOnLaunch

[sync-by-unito]

See #195

Similar to Realm Java we should expose the option of compacting on launch.

This also involves exposing an interface like

public interface CompactOnLaunchCallback {

    /**
     * This method determines if the Realm file should be compacted before opened and returned to the user. It is passed
     * the total file size (data + free space) and the bytes used by data in the file.
     *
     * @param totalBytes the total file size (data + free space).
     * @param usedBytes the total bytes used by data in the file.
     * @return {code true} to indicate an attempt to compact the file should be made. Otherwise, the compaction will be
     * skipped.
     */
    boolean shouldCompact(long totalBytes, long usedBytes);
}

And with a similar configuration with a similar API to Realm Java

SyncConfiguration.Builder(user, partition)
 .compactOnLaunch() // Default policy
 .compactOnLaunch(callback: CompactOnLaunchCallback)
 .build()

[cmelchior]

Added to both RealmConfiguration and SyncConfiguration here: #658