Tables in the table definition cache are keeping a cache buffer for blob
fields which can consume a lot of memory.
    
This patch introduces a maximum size threshold for these buffers.


sql/sql_base.cc:
  Added function free_field_buffers_larger_than to reclaim memory from blob
  field buffers too large to be cached.
sql/table.cc:
  Added function free_field_buffers_larger_than