Memcached ops/sec drop if a UPR consumer is running on the cluster

Memcached ops/sec can drop to 500 ops/sec from 10K ops/sec if a UPR consumer is subscribing to the cluster. This is lot of free RAM, CPU etc while this is happening.

Steps to reproduce:

1. Install couchbase-server-enterprise_x86_64_3.0.0-350-rel.deb on single node (amazon m1.xlarge, 15G RAM, 4 ecu)

2. Create default bucket

3. Start a UPR consumer which subscribes for all 1024 vbuckets for mutations.

4. Start front-end load using 2 mcsoda clients from 2 separate EC2 nodes (500K items, 30% sets, 70% gets)
client1:
./lib/perf_engines/mcsoda.py membase://10.147.4.31:8091 vbuckets=1024 doc-gen=0 doc-cache=0 ratio-creates=1 ratio-sets=0.3 min-value-size=256,512 max-items=500000 exit-after-creates=1 prefix=70one

client2:
./lib/perf_engines/mcsoda.py membase://10.147.4.31:8091 vbuckets=1024 doc-gen=0 doc-cache=0 ratio-creates=1 ratio-sets=0.3 min-value-size=256,512 max-items=500000 exit-after-creates=1 prefix=70two

This generates roughly 18k ops/sec as seen in screenshot normal_cluster.png

5. Repeat Step4 after changing the prefix 2-3 times. One of the clients starts getting very slow response as can be seen in througput_drop.png. The ops/sec drop to 500 from 9-10k.

6. Kill the UPR consumer while client is experiencing low ops/sec. Instantly the ops/sec surge back to 10k ops/sec as can be seen in after_killing_upr_consumer.png.

The UPR consumer is only taking 30% cpu and no memory. Its not doing any network activity as well(its running on the KV node).