CurrentSetting change + yaml

overridden neverConstant
added test to yaml
current_* functions now neverConstant

changed all other funds with current in name to return true for
neverConstant
changed noArgExpression’s default neverConstant() to return false , I
am assuming that this buys us something for the functions that now
return false for never constant
cleanup

cleanup unneeded yaml queries
fixing

changed default back to return true as only three of the functions are
constant
fixing NoArgFuncs

Now only functions that return false for neverConstant are PI,
SHORT_SERVER_VERSION and SYSTEM_USER
yaml fix

removed repeated line
merge JerettS: Current setting constant bug

Merge pull request #1428 from JerettS/sql-layer/CurrentSettingConstantBug
https://github.com/FoundationDB/sql-layer/pull/1428