More refinements for tests plus new VFS object and R functions

[eddelbuettel]

This PR collects a few things from the last few days en route to 0.7.2

• refines some earlier work on setting core and thread counts
• updates the quickstart_sparse example
• make tiledb_get_context() exported and visible (earlier oversight)
• updates the test matrix to 2.0.6
• renames the C++ level vfs* function libtiledb_vfs_* in line with all other C++ functions
• adds R level functions tiledb_vfs_* as well as new S4 object creator for VFS
• adds tests for the VFS functions

[ihnorton]

one question, otherwise lgtm