Storage engine independent support for column compression.

TINYBLOB, BLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT,
VARCHAR and VARBINARY columns can be compressed.

New COMPRESSED column attribute added:
COMPRESSED[=<compression_method>]

System variables added:
column_compression_threshold
column_compression_zlib_level
column_compression_zlib_strategy
column_compression_zlib_wrap

Status variables added:
Column_compressions
Column_decompressions

Limitations:
- the only supported method currently is zlib
- CSV storage engine stores data uncompressed on-disk even if COMPRESSED
  attribute is present
- it is not possible to create indexes over compressed columns.