Enhancement: FixedLenKeyIndex: remove same bytes in the middle

[rockeet]

Now we have removed common prefix len in FixedLenKeyIndex.

In some cases, there are many common bytes in the middle of keys, for example:

In MyTopling, secondary keys has primary key at the end, common prefix in primary keys are the middle bytes of secondary keys, these bytes can be removed.

CREATE TABLE a (
  id BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  ts DATETIME,
  INDEX(ts)
);

Encoded key of secondary index ts has form ts(DATETIME) id(BIGINT), in which there are many zero leading bytes in id.

[rockeet]

Realized by FixedLenHoleIndex, commits:

topling/topling-zip_table_reader@e0b5fd7
topling/topling-zip_table_reader@4d6283e
topling/topling-zip_table_reader@b23e0df