remove MENSHEN_ALLOW_GATEWAY_LIST and MENSHEN_ALLOW_BRIDGE_LIST after these environment variables has been removed from menshen

I ran: go get -u ./...

On the client side (bitmask-core) and on the server side
(obfsvpn) we should use the same version of github.com/xtaci/kcp-go.
If not the connection fails, see:
obfsvpn#69 (comment 1238664)
We now use the dialer returned by our obfsvpn package. We only need to
use/update the package instead of checking both repos use the exact same
version of the kcp library.

instead of embedding BitmaskCoreMobile interface into BitmaskMobile, try to assign an empty BitmaskMobile struct to BitmaskCoreMobile typed variable in order to ensure on compile time that BitmaskMobile implements all methods of BitmaskCoreMobile

move provider.json required in unit test to appropriate testdata directory, adapt code reading the file

Revert adding api subdomain to v5 bootstrapping endpoints. This reverts a temporary workaround which existed so that bitmask-core could be used against live backends, the proper fix was done on the backend side instead

add tests that verify the deserialization of a provider's AllowAnonymous and AllowRegistration fields

This interface allows us to easily mock BitmaskMobile methods in Java in unit tests,
without the need to load the actual golang shared library.

comment out tests that broke due to a temporary work-around to test against current staging environment. Adding hints to revert these changes once menshen#63 has been fixed

revert sending model objects via the mobile FFI, instead sent json formatted strings. integrating apps will need to deserialize them on their own, e.g. by using the swagger code generator of for their particular language. This way we can work around gomobie's lack of support for slices and maps of strucs which the menshen swagger model classes contain