IPv6 paths not correctly parsed in hdfs connectToPath

> Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev

### Expected behavior
hdfs://[2a05::]:port/ should work

### Actual behavior
"Bad uri for hdfs"

### Steps to reproduce the behavior
Look at the source code in rocksdb/hdfs/env_hdfs.h, it does a naive split. Does not parse IPv6 addresses correctly.

~~~
std::vector parts;
split(hostport, ':', parts);
if (parts.size() != 2) {
throw HdfsFatalException("Bad uri for hdfs " + uri);
~~~