fix(tianmu): fix specified key was too long; max key length is 255 bytes(#228)

[lujiashun]

fix(tianmu): fix specified key was too long; max key length is 255 bytes(#228)

(1). implement tianmu's virtual function max_supported_key_part_length; (2). enlarge max_supported_key_length's return value from 1024 to 16*1024;

Summary about this PR

Issue Number: close #228
1、tianmu engine limit the max_supported_key_part_length to 255 which is the default storage engine value ；
1.1、 innodb is key-part-length default is 768-1; if set the innobase_large_prefix variables to ture(default is false), key-part-length can be 3072;
1.2. rocksdb's key-part-length default is 767(768-1); if set rocksdb_large_prefix variables to ture(default is false) , key-part-length can be 3072;
1.3、add a system variable tianmu_large_prefix(default values is false,and key-part-length is 767).if the system variable is true, key-part-length is 3072
2、meanwhile tianmu engine limit max_supported_key_length to 1024， while innodb is 3500.
refer to myrock5.6, the max_supported_key_length is 16* 1024， adapt this value。

Tests Check List

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Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features

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[RingsC]

LGTM

[adofsauron]

LGTM