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.
fix(README): use the new etcd-dev mailing list

We moved to google groups, update the URLs
fix(CONTRIBUTING): point to new mailing list
fix(raft_server_stats.go) protect raft stats by a lock
fix(registry.go) protect the peer and proxy map in the registry by lock
fix(store.go) send event to watcher after we finish modifying it
fix(discovery_test.go) protect the garbageHandler by a lock
fix(internal_version_test.go) protect the checkedVersion by a lock
Revert "Revert "fix(server): only set NOCOW for log file""

This reverts commit 9540575690db92a362f66d9aa4f2671265b87eb1.

Conflicts:
	etcd/etcd.go
tests(test.sh) open race option
tests(snapshot_test) loose the timing assumption for snapshot test
Test run slowly on drone after open race option.
make necessary changes
fix(peer join) fix wrong join command redirection
1. We use PUT request to do a V2 join. So we should redirect a PUT request rather than a POST.
2. /admin only accept V2Join request. Send out V2Join instead of V1Join.