VBSnapshotTask uninitialised priority

A mistake in https://github.com/couchbase/ep-engine/commit/6403bc0e8bbd7e94bb03672f505d99ff68d56c36 means the priority value is not initialised.

This was spotted by valgrind when merging this commit up to Watson, presumably valgrind is not ran on sherlock, or the code path is not tested.

Issue is here, the base class priority field doesn't get initialised, yet it is read in the run function

• Not initialised here http://src.couchbase.org/source/xref/sherlock/ep-engine/src/tasks.h#268
• Read here http://src.couchbase.org/source/xref/sherlock/ep-engine/src/tasks.cc#127

This issue means the task will run at HIGH or LOW, but not guaranteed at the requested priority.