Previously, tags had a `shouldCopy` flag to indicate if those tags
referenced an underlying buffer and should be copied to allow GC.
Unfortunately, this prevented tags from being copied that were
created and referenced the mmap which caused segfaults.
This change removes the `shouldCopy` flag and replaces it with a
`forceCopy` argument in `CreateSeriesIfNotExists()`. This allows
the write path to indicate that tags must be cloned on insert.
go 1.5 was being used to develop the query engine branch, but we aren't
using 1.5 for master at the moment. This fixes issues that go vet brings
up in 1.4 that don't exist in 1.5.