Initialize recursive protobuf fields more efficiently.

[copybara-service]

Initialize recursive protobuf fields more efficiently.

Currently, a field (F0) is not initialized if there are subfields of the form F0 -> F1 -> ... -> Fs -> ... -> Fn -> Fs. This means that for a huge protobuf field that has a recursive subfield deep in its definition, the whole field is not initialized. And later, even when F0 is initialized, F1 won't get initialized, etc. This could be very inefficient. To avoid this, we define "recursion breaker fields". For example, "Fs" becomes a recursion breaker. Then, all fields up to Fs are initialized. And later when Fs gets initialized, all Fs -> ... -> Fn get initialized.

This CL consists of the following changes:

• IsProtoRecursive deals with infinite recursions only.
• IsFinitelyRecursive is replaced with IsRecursionBreaker which is implemented separately.
• Having separate strategy for handling of infinite recursion and finite recursion (through recursion breakers), we need to explicitely avoid initialization and mutation of infinitely recursive fields.