Change some settings defaults for better behaviour out of the box

- CheckingMemUsageSize (16 MiB -> 32 MiB): a 16 MiB increase in memory consumption seems worthwhile for a nice performance boost in most cases.
- DiskCacheSize (64 MiB -> Auto): auto yields the best performance without committing to a huge fixed value.
- UseRandomPort (false -> true): The initial port chosen by qBittorrent may clash with something else the user already has that is aways using that port (low probability, but still). Thus, qBittorrent will always fail listening on that port, causing unexpected problems for the user. Users who know they want a fixed port will go to the settings anyway.

src/base/bittorrent/session.cpp

@@ -374,8 +374,8 @@ Session::Session(QObject *parent)
     , m_announceToAllTiers(BITTORRENT_SESSION_KEY("AnnounceToAllTiers"), true)
     , m_asyncIOThreads(BITTORRENT_SESSION_KEY("AsyncIOThreadsCount"), 4)
     , m_filePoolSize(BITTORRENT_SESSION_KEY("FilePoolSize"), 40)
-    , m_checkingMemUsage(BITTORRENT_SESSION_KEY("CheckingMemUsageSize"), 16)
-    , m_diskCacheSize(BITTORRENT_SESSION_KEY("DiskCacheSize"), 64)
+    , m_checkingMemUsage(BITTORRENT_SESSION_KEY("CheckingMemUsageSize"), 32)
+    , m_diskCacheSize(BITTORRENT_SESSION_KEY("DiskCacheSize"), -1)
     , m_diskCacheTTL(BITTORRENT_SESSION_KEY("DiskCacheTTL"), 60)
     , m_useOSCache(BITTORRENT_SESSION_KEY("UseOSCache"), true)
 #ifdef Q_OS_WIN
@@ -432,7 +432,7 @@ Session::Session(QObject *parent)
     , m_isBandwidthSchedulerEnabled(BITTORRENT_SESSION_KEY("BandwidthSchedulerEnabled"), false)
     , m_saveResumeDataInterval(BITTORRENT_SESSION_KEY("SaveResumeDataInterval"), 60)
     , m_port(BITTORRENT_SESSION_KEY("Port"), -1)
-    , m_useRandomPort(BITTORRENT_SESSION_KEY("UseRandomPort"), false)
+    , m_useRandomPort(BITTORRENT_SESSION_KEY("UseRandomPort"), true)
     , m_networkInterface(BITTORRENT_SESSION_KEY("Interface"))
     , m_networkInterfaceName(BITTORRENT_SESSION_KEY("InterfaceName"))
     , m_networkInterfaceAddress(BITTORRENT_SESSION_KEY("InterfaceAddress"))