Removed valgrind warnings when using not aligned bitmap buffer size
Added setting of thread_stack


include/my_bitmap.h:
  Added bitmap_buffer_size()
sql/handler.cc:
  use bitmap_buffer_size()
sql/opt_range.cc:
  use bitmap_buffer_size()
sql/sql_insert.cc:
  Use bitmap_buffer_size()
sql/sql_parse.cc:
  Added setting of thread_stack when taking thread from thread cache
sql/sql_plugin.cc:
  Set thread_stack