Avoid duplicated GeneratedMethodAccessor when reflect method invoked from different threads

Reflection method is firstly invoked with JNI, after invocation count exceeds threshold specific GeneratedMethodAccessor is created to optimize invocation speed.

In current implementation, there is no lock/synchornization between multiple threads create same GeneratedMethodAccessor in same NativeMethodAccessorImpl object. Multiple GeneratedMethodAccessor could be created spontanously, this wastes CPU and multiple creations could cause a lot contention in runtime define_instance_class.

Above scenario could be happened when using protobuf generated classes at startup phase, especially with option "optimize_for = CODE_SIZE". Multiple threads invoke same reflect methods to send/receive data at startup time. Redudant GeneratedMethodAccessor creations cause startup slow and most time is spent in runtime define_instance_class lock contention.

Attached is a simplied testcase, create 10k reflection methods and invoked in 100 threads. On 8 core linux X86, avoid duplicated GeneratedMethodAccessor reduce runtime from 12s to < 1.5s.