Allow setting chroot for a connection

[spenczar]

Added simple chroot support, requested in #32.

Conn gets a SetChroot function which fiddles with the path of any requests which use a path.

It has a known bug: if the Chroot'd connection is asked to work on paths that look like the chroot itself, it will do things to the wrong directory. In particular, if you do this:

conn.SetChroot("/a/b")
conn.Create("/a/b/c", []byte{}, 0, WorldACL(PermAll))

Then you will not get a node written to "/a/b/a/b/c", as you might expect - it will go to "/a/b/c". This is unfortunate, but hard to avoid.

Also, we don't currently fiddle with response Paths, which is probably flat-out wrong... but I'll wait to hear whether this is a direction worth moving in before adding that.

[spenczar]

I've added fiddling of response paths, and quite a few tests. The build failure is just from unsafe startup of the test cluster. I'll submit a fix for that in a separate PR.

[thorhs]

I would like this feature very much. To me, the client should not be aware that it has been chrooted, that is, all operations should return paths relative to the chroot, not the actual root.

Also, the library should accept connection strings in the same format as the normal ZK library:

connectString - comma separated host:port pairs, each corresponding to a zk server. e.g. "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002" If the optional chroot suffix is used the example would look like: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a" where the client would be rooted at "/app/a" and all paths would be relative to this root - ie getting/setting/etc... "/foo/bar" would result in operations being run on "/app/a/foo/bar" (from the server perspective)

[spenczar]

@thorhs , just to be clear - the current diff successfully hides chrooting from the client. It doesn't parse connection strings in any particularly clever way, though. I can add that, although I want to wait for #76 to get in so that I can run tests.

[zellyn]

I've since added options to the Connect(...) method, so you should probably just make a WithChroot(string) connOption method for setting this.