Pass (parts of) environment into execution part of UNIX_TIMESTAMP and FROM_UNIX_TIME functions

Execution of UNIX_TIMESTAMP and FROM_UNIX_TIME functions on the PrimProc does not have access to execution's environment variables needed for correct execution of these functions.

We should use fDynamicFunctor in FunctionColumn for these two functions and add state to them, so that we can pass execution environment settings to them. We need to recognize these functions in the getSelectPlan() execution (ha_mcs_execplan.cpp) and set their state there.

A good examples of that is encode or {{rand}}functions.