PHOENIX-6665 changed column name for CURRENT seqence values

Due to changes in execution, I had to change the string value returned by SequenceValueExpression (which is used as the column name)

However, as the CURRENT keyword does not take a parameter, the column name should have remained the same.

Fix the column names, so that we return 

CURRENT VALUE FOR MYSEQUENCE

instead of 

CURRENT[1] VALUE(S) FOR MYSEQUENCE

again.

The column names for NEXT VALUE(S) were also changed, but that cannot be avoided.