TabletMeta's _preferred_rowset_type is not initialized after object constructing and
may be a random value, and this field is not updated when create ALPHA_ROWSET tablet,
and it will not be serialized into pb in this case. So if cloning an ALPHA_ROWSET
tablet from another BE, this new created local tablet's _preferred_rowset_type field
may be random as BETA_ROWSET and can not be overwrote after cloned, then new input
rows will be wrote as BETA_ROWSET format which is not we expect.
This patch fix this bug by giving _preferred_rowset_type a default value and updating
this field when create any type of tablet, and add an unit test and related overwrite
equal operator functions.
3b6a781862