Accessor style for optional fields.

This CL implements the 'optional_field_style=accessors' option.
All optional fields will now be 1 Java field and 1 bit in a shared
bitfield behind get/set/has/clear accessor methods. The setter
performs null check for reference types (Strings and byte[]s).

Also decentralized the clear code generation.

Change-Id: I60ac78329e352e76c2f8139fba1f292383080ad3