MB-8102: Switched from using select(..) to poll(..) within mcs_io_read

This prevent segfaults when more than 1024 fd's are used.

Change-Id: I5513af3fdeca6d807d14d5977e9ca2cb5b2c8719
Reviewed-on: http://review.couchbase.org/30345
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>