Separate UIDs and Revisions in main databases

Summary:
- Change revision type from `qint64` to `size_t` for LMDB in a couple of places (LMDB supports `unsigned int` or `size_t` which are `long unsigned int` on my machine)
- Better support for database flags (duplicate, integer keys, integer values for now but is extensible)
- Main databases' keys are now revisions
- Some databases switched to integer keys databases:
  - Main databases
  - the revision to uid mapping database
  - the revision to entity type mapping database
- Refactor the entity type's `typeDatabases` method (if in the future we need to change the main databases' flags again)
- New uid to revision mapping database (`uidsToRevisions`):
  - Stores all revisions (not uid to latest revision) because we need it for cleaning old revisions
  - Flags are: duplicates + integer values (so findLatest finds the latest revision for the given uid)

~~Problems to fix before merging:~~
All Fixed!

- ~~Sometimes Sink can't read what has just been written to the database (maybe because of transactions race conditions)~~
  - ~~Most of the times, this results in Sink not able to find the uid for a given revision by reading the `revisions` database~~
- ~~`pipelinetest`'s `testModifyWithConflict` fails because the local changes are overridden~~

~~The first problem prevents me from running benchmarks~~

Reviewers: cmollekopf

Tags: #sink

Differential Revision: https://phabricator.kde.org/D14974

1 files changed, 14 insertions, 0 deletions

diff --git a/common/utils.h b/common/utils.h
index 7066d79..8565f17 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -26,6 +26,20 @@ namespace Sink {
 
 QByteArray createUuid();
 
+// No copy is done on this functions. Therefore, the caller must not use the
+// returned QByteArray after the size_t has been destroyed.
+const QByteArray sizeTToByteArray(const size_t &);
+size_t byteArrayToSizeT(const QByteArray &);
+
+template <typename T>
+static QByteArray padNumber(T number);
+
+template <>
+QByteArray padNumber<size_t>(size_t number)
+{
+    return padNumber<qint64>(number);
+}
+
 template <typename T>
 static QByteArray padNumber(T number)
 {