rgw: make RGWEnv return a const ref. to its map

rgw: make RGWEnv return a const ref. to its map

We already have a public method `set` for setting values in the RGW env
map, making the map as a read only. In addition:

- Added const to other  methods in `RGWEnv` which are getters
- `req_info::init_meta` also reused the same `iter` name for an internal
  find, changed the var name
- `add_grants_headers` now accepts an RGWEnv instead of the map
- use range based for loops wherever the code was changed
- req_info holds a ptr to a const RGWEnv, since we seem to only read the
  values from it

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>