[Server] Harden private chat history identity matching

Match both participants by account id and name so a federated user's remote id cannot collide with a local account, and stop storing foreign ids in the local chat log. Extract the paging assembly into a pure helper and cover the whole-second boundary rules with gtests.

tests/CMakeLists.txt

@@ -426,6 +426,19 @@ cockatrice_add_test(
   LABELS
   server
 )
+cockatrice_add_test(
+  NAME
+  private_chat_paging_test
+  SOURCES
+  ${CMAKE_SOURCE_DIR}/servatrice/src/private_chat_paging.cpp
+  private_chat_paging_test.cpp
+  LIBS
+  libcockatrice_protocol
+  INCLUDE_DIRS
+  ${CMAKE_SOURCE_DIR}/servatrice/src
+  LABELS
+  server
+)
 cockatrice_add_test(
   NAME
   latency_tracker_test