Rename map to struct in schema definition

Drill maps concept is more close to Struct concept. Good write up by Paul about this:
https://github.com/paul-rogers/drill/wiki/Drill-Maps. Internally such objects are represented by MapVector which in future will be renamed to StructVector (similar was done in Arrow project). Drill also does not support true maps, which in future will be also implemented with proper naming (MapVector).

Nevertheless, internal map / struct presentation does not affect users plus before schema provisioning they did not deal with this concept at all.
Initially map complex type in Schema Provisioning schema was represented with map keyword followed by angle brackets: col1 map<m1 int, m2 int>. Given second thought to map notion, looks like it's better to use struct in schema definition and internally convert it to map metadata (again which eventually will be renamed to struct). Such approach would ease map to struct renaming (since it will be done internally) and users will be using struct naming as before. Once Drill will implement true map, schema provisioning will allow map type for schema.