BUG#21051: RESET QUERY CACHE very slow when query_cache_type=0

There were two problems: RESET QUERY CACHE took a long time to complete
and other threads were blocked during this time.

The patch does three things:
  1 fixes a bug with improper use of test-lock-test_again technique.
      AKA Double-Checked Locking is applicable here only in few places.
  2 Somewhat improves performance of RESET QUERY CACHE.
      Do my_hash_reset() instead of deleting elements one by one.  Note
      however that the slowdown also happens when inserting into sorted
      list of free blocks, should be rewritten using balanced tree.
  3 Makes RESET QUERY CACHE non-blocking.
      The patch adjusts the locking protocol of the query cache in the
      following way: it introduces a flag flush_in_progress, which is
      set when Query_cache::flush_cache() is in progress.  This call
      sets the flag on enter, and then releases the lock.  Every other
      call is able to acquire the lock, but does nothing if
      flush_in_progress is set (as if the query cache is disabled).
      The only exception is the concurrent calls to
      Query_cache::flush_cache(), that are blocked until the flush is
      over.  When leaving Query_cache::flush_cache(), the lock is
      acquired and the flag is reset, and one thread waiting on
      Query_cache::flush_cache() (if any) is notified that it may
      proceed.


include/mysql_com.h:
  Add comment for NET::query_cache_query.
sql/net_serv.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query if query cache is used.
  Do not access net->query_cache_query without a lock.
sql/sql_cache.cc:
  Fix bug with accessing query_cache_size, Query_cache_query::wri and
  thd->net.query_cache_query before acquiring the lock---leave
  double-check locking only in safe places.
  Wherever we check that cache is usable (query_cache_size > 0) we now
  also check that flush_in_progress is false, i.e. we are not in the
  middle of cache flush.
  Add Query_cache::not_in_flush_or_wait() method and use it in
  Query_cache::flush_cache(), so that threads doing cache flush will
  wait it to finish, while other threads will bypass the cache as if
  it is disabled.
  Extract Query_cache::free_query_internal() from Query_cache::free_query(),
  which does not removes elements from the hash, and use it together with
  my_hash_reset() in Query_cache::flush_cache().
sql/sql_cache.h:
  Add declarations for new members and methods.
  Make is_cacheable() a static method.
  Add query_cache_init_query() function.
sql/sql_class.cc:
  Use query_cache_init_query() for initialization of
  NET::query_cache_query.

include/mysql_com.h

@@ -210,7 +210,13 @@ typedef struct st_net {
   char last_error[MYSQL_ERRMSG_SIZE], sqlstate[SQLSTATE_LENGTH+1];
   unsigned int last_errno;
   unsigned char error;
+
+  /*
+    'query_cache_query' should be accessed only via query cache
+    functions and methods to maintain proper locking.
+  */
   gptr query_cache_query;
+
   my_bool report_error; /* We should report error (we have unreported error) */
   my_bool return_errno;
 } NET;