20-30% performance degradation on append-heavy workload compared to 2.5.1

When running an append-heavy workload (modelling a social network address book, see below) the performance of CB has dropped from ~100K ops down to 50K ops compared to 2.5.1-1083 on OS X.

Edit: I see a similar (but slightly smaller - around 40% degradation on Linux (Ubuntu 14.04) - see comment below for details.

== Workload ==

revAB_sim - generates a model social network, then builds a representation of this in Couchbase. Keys are a set of phone numbers, values are lists of phone books which contain that phone number. (See attachment).

Configured for 8 client threads, 100,000 people (documents).

To run:

• pip install networkx
• Check revAB_sim.py for correct host, port, etc
• time ./revAB_sim.py

== Cluster ==

1 node, default bucket set to 1024MB quota.

== Runtimes for workload to complete ==

1. 1. CB-2.5.1-1083:

~107K op/s. Timings for workload (3 samples):

real 2m28.536s
real 2m28.820s
real 2m31.586s

1. 1. CB-3.0.0-918

~54K op/s. Timings for workload:

real 5m23.728s
real 5m22.129s
real 5m24.947s