Add HTTP POST/PUT/DELETE support for web servers in datanodes

Currently, HDFS files can be read from datanodes by their web servers, but cannot be written or deleted. This add HTTP POST/PUT/DELETE support for HDFS. In requests, HTTP Header must contain Content-Length and Content-Type should NOT be "application/x-​www-form-urlencoded". In POST or PUT requests, target data need to be directly stored in HTTP body.