chore(build): set build path to gopath/

With this, pkg/ could be used as normal directory, and it doesn't need
to gitignore pkg/, which shields pkg/http and pkg/strings.
feat: set NOCOW for log directory when in btrfs
chore(pkg/fs): rewrite comment
fix(server): only set NOCOW for log file
fix(fs): rewrite test to avoid side effect
Revert "fix(server): only set NOCOW for log file"

This reverts commit 1eff547af61c2453b106f65691f928ddf8088a6b.
fix(fs): make check on NOCOW
chore(btrfs): rename fs to btrfs

This is because the actions is specific for btrfs.
Revert "Revert "fix(server): only set NOCOW for log file""

This reverts commit 9540575690db92a362f66d9aa4f2671265b87eb1.

Conflicts:
	etcd/etcd.go
make necessary changes
Merge pull request #705 from unihorn/61

feat: set NOCOW for log directory when in btrfs