Delete and re-add libpqxx-7.7.3 due to weird corruption.

ext/libpqxx-7.7.3/include/pqxx/doc/thread-safety.md

@@ -1,29 +0,0 @@
-Thread safety                    {#thread-safety}
-=============
-
-This library does not contain any locking code to protect objects against
-simultaneous modification in multi-threaded programs.  Therefore it is up
-to you, the user of the library, to ensure that your threaded client
-programs perform no conflicting operations concurrently.
-
-Most of the time this isn't hard.  Result sets are immutable, so you can
-share them between threads without problem.  The main rule is:
-
-@li Treat a connection, together with any and all objects related to it, as
-a "world" of its own.  You should generally make sure that the same "world"
-is never accessed by another thread while you're doing anything non-const
-in there.
-
-That means: don't issue a query on a transaction while you're also opening
-a subtransaction, don't access a cursor while you may also be committing,
-and so on.
-
-In particular, cursors are tricky.  It's easy to perform a non-const
-operation without noticing.  So, if you're going to share cursors or
-cursor-related objects between threads, lock very conservatively!
-
-Use `pqxx::describe_thread_safety` to find out at runtime what level of
-thread safety is implemented in your build and version of libpqxx.  It
-returns a `pqxx::thread_safety_model` describing what you can and cannot rely
-on.  A command-line utility `tools/pqxxthreadsafety` prints out the same
-information.