Initial support for UPR.

Summary of changes:

provides a feed similar to TAP interface to receive mutations. Support UPR restart. This implementation may change based on secondary index requirements.

All upr additions are confined to upr.go, uprtrans.go, uprtrans_test.go. These
files are newly added for upr support.

Added example programs to check UPR client operations. These programs are
under "examples/" directory and confined to individual files.

Fixed a small bug in pools.go