Delete map keys

[scommisso]

Either #update() needs to support deleting map keys (maybe triggered by null values?), or #remove() semantics need to be updated to support deleting map keys.

Since we already have validation that #update() cannot set map keys to null, I propose that null values in here will generate a DELETE <mapFieldName>["<mapNullPropName>"] FROM <entityName> WHERE <conditionals> statement.

Right now, to "delete" a map key, we are required to set it to some sort of empty value. This is fine for most maps, but causes problems with a map<text, JSON>.

[jcrugzz]

@scommisso want to take a stab at this one when you have the bandwidth?