entry: use AdditionalData2 for serialization

Prefer the `map<std::string, std::string>` for serialization of
metadata over the `vector<std::string>` format.  Add backwards
compatible parsing of the `vector<std::string>` format.

Tested: Loaded a "version 5" entry.  Created a new entry and verified
it was "version 6" in the binary dump.  Reloading the daemon restores
both version 5 and 6 objects.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I39b8a4099ad61161d52c3784d6325df7da031011
diff --git a/config/config.h.meson b/config/config.h.meson
index f852bab..0c3bb6d 100644
--- a/config/config.h.meson
+++ b/config/config.h.meson
@@ -34,7 +34,8 @@
 static constexpr auto FIRST_CEREAL_CLASS_VERSION_WITH_UPDATE_TS = "3";
 static constexpr auto FIRST_CEREAL_CLASS_VERSION_WITH_EVENTID = "4";
 static constexpr auto FIRST_CEREAL_CLASS_VERSION_WITH_RESOLUTION = "5";
-static constexpr size_t CLASS_VERSION = 5;
+static constexpr auto FIRST_CEREAL_CLASS_VERSION_WITH_METADATA_DICT = "6";
+static constexpr size_t CLASS_VERSION = 6;
 
 static constexpr bool LG2_COMMIT_DBUS = @lg2_commit_dbus@;
 static constexpr bool LG2_COMMIT_JOURNAL = @lg2_commit_journal@;