Fixes https://github.com/grpc/grpc/issues/2194 .

Part of fixing https://github.com/grpc/grpc/issues/2192

And free ops_array that was leaking.

Introduced grpc_byte_buffer_init instead. It's now the user's responsibility to
manage memory.

In library projects (cf. app projects) the main bundle is nil. `NSBundle+bundleForClass:` works in both types of projects.

Also makes the library load the certificates only once.