modify memory requirement calculation.

* docs: add R example source code

* test: check if RJDBC was installed

* test: add R test scripts

* test: chmod of test_R.sh

* test: fix test_go.sh; add R sample to document

* test: add test scripts to cases.task

* test: fix errors caused by  tset_python.sh and test_java.sh

* test: fix  test_python.sh

* test: modify java test case and improve opentsdb-json.mdx

* test: fix test_R.sh