store: remove prev value before AtomicPut
store: remove prev value before AtomicPut

libkv etcd implementation populates both prevIndex and prevValue to the
v2 api. This is causing problems with big values (since it puts the
prevValue in the PUT url).

This patch removes the KVPair Value before invoking the AtomicPut.

sorintlab/stolon,smkingsoft/stolon,sorintlab/stolon,siepkes/stolon,sgotti/stolon,siepkes/stolon,sgotti/stolon,smkingsoft/stolon