[FTS] moss store potential data loss with parallel store.Persist() invocations

Say we have 2 go routines writing data into mossStore,
go routine 1: ExecuteBatch() with Segment X, takes snapshot ( X ) & calls store.Persist( X )
go routine 2: ExecuteBatch() with Segment Y, takes snapshot (X+Y) & calls store.Persist(X+Y)

Now go routine 1's Persist ( X ) is called first grabs the store lock, builds new store footer and releases the store lock here

https://github.com/couchbase/moss/blob/master/store.go#L136

then goes ahead and issues I/O to actually persist the segment ( X ) 

go routine 2 calls Persist (X+Y) grabs the lock released by routine 1, builds store footer footer and is now persisting segment ( X + Y).

Now due to I/O race, go routine 2's write somehow completes first and it manages to grab the second lock here

https://github.com/couchbase/moss/blob/master/store.go#L159

Now the footer has X + Y and everything is good.. until...

go routine 1 completes its I/O and writes out its footer which only has segment X.

This results in data loss of segment Y in case there is a shutdown or crash immediately after as far as go routine 2 is concerned.

Simple fix might just be to hold on to the store lock until the footer is persisted, but need to evaluate the side effects on things like snapshots etc.

thanks