Get option recv_min_bytes correctly

Daniel Owen found the bug when working on cbbacup performance issue.

During my testing I also discovered a bug. In pump_tap.py function def _init_(self, opts, spec, source_bucket, source_node,source_map, sink_map, ctl, cur): we are setting recv_min_bytes as follows:

self.recv_min_bytes = int(getattr(opts, "recv_min_bytes",4096))

recv_min_bytes is never found and so setting the extra parameter has no effect. The reason it is not found is because it is in the extra section. i.e.
print getattr(opts, "extra") returns the following

{'max_retry': 10.0, 'rehash': 0.0, 'data_only': 0.0, 'nmv_retry': 1.0, 'conflict_resolve': 1.0, 'cbb_max_mb': 100000.0, 'try_xwm': 1.0, 'batch_max_bytes': 400000.0, 'report_full': 2000.0, 'batch_max_size': 1000.0, 'report': 5.0, 'design_doc_only': 0.0, 'recv_min_bytes': 8192.0}

I believe it should be replaced with the following

self.recv_min_bytes = int(self.opts.extra['recv_min_bytes'])