Add void* arguments to request_new and reply_new evrpc hooks

Add void* arguments to request_new and reply_new evrpc hooks

This makes evprc setup more extensible, and helps with Shuo Chen's
work on implementing Google protocol buffers rpc on top of Libevent 2
evrpc.

This patch breaks binary compatibility with previous versions of
Libevent, since it changes struct evrpc and the signature of
evrpc_register_generic().  Since all compliant code should be calling
evrpc_register_generic via EVRPC_REGISTER, it shouldn't break source
compatibility.

(Code by Shuo Chen; commit message by Nick)