Returning "auto" makes it harder to understand what the function is returning,
and impossible to de-inline.
Return a vector of pointers instead. The caller should iterate immediately, in
any case, and since the previous return value was a range of references to const
unique_ptrs, nothing else could be done with it anyway.
The extensions class reaches into cql's property_definitions class to grab
a map<sstring, sstring> type. This generates a few unneeded dependencies.
Reduce dependencies by defining the map type ourselves; if cql's property_definitions
changes in an incompatible way, it will have to adapt, rather than the extensions
class.
Allows extensions/config listeners to potentially augument
(system) schemas at boot time. This is only useful for schemas
who do not pass through system_schema tables.
