Rework datastores

DatastoreAccess now has "targets". The target determines what datastores
are used for writing and reading data. There are three targets:
  Operational:
  - reads from operational, writes to running
  Startup:
  - reads from startup, writes to startup
  Running:
  - reads from running, writes to running

(the datastores types above correspond to the NMDA datastore types)

The need to define targets instead of just calling them "datastores" is
that the operational target doesn't actually use the operational
datastore for both writing and reading.

Change-Id: Iffb7034c27aba42d10d715e23a2c970031b9bd1b
diff --git a/src/sysrepo_access.hpp b/src/sysrepo_access.hpp
index 55e8ac6..7a02d8b 100644
--- a/src/sysrepo_access.hpp
+++ b/src/sysrepo_access.hpp
@@ -27,7 +27,7 @@
 class SysrepoAccess : public DatastoreAccess {
 public:
     ~SysrepoAccess() override;
-    SysrepoAccess(const Datastore datastore);
+    SysrepoAccess();
     [[nodiscard]] Tree getItems(const std::string& path) const override;
     void setLeaf(const std::string& path, leaf_data_ value) override;
     void createItem(const std::string& path) override;