MB-24990: Prevent lock inversion during stats & file removal
MB-24990: Prevent lock inversion during stats & file removal

Deadlock caused when:
Stats go routine:
================
moss.(*FileRef).FetchRefCount+0x38 file.go:134
moss.(*Store).allFiles+0x88e store.go:343
 (Has the store lock tries to grab FileRef lock)

File Removal go routine
================:
 moss.(*Store).removeFileOnClose.func1+0x68 store.go:315
 moss.(*FileRef).DecRef+0x12b file.go:109
 moss.(*mmapRef).DecRef+0xee mmap.go:55
 moss.SegmentLocs.DecRef+0xc9 store_api.go:188
 moss.(*Footer).DecRef+0x79 store_footer.go:368
 moss.(*Footer).Close+0x2a store_footer.go:352
 (Has the FileRef lock and tries to grab store lock)

Change-Id: I772dd292bac4e3b7f821e08aceea5a9e6a494697
Reviewed-on: http://review.couchbase.org/79909
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Reviewed-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

steveyen/moss,couchbaselabs/moss,steveyen/moss