Performance improvements, easier profiling, add `--hhvm-4.2-to-4.3` migration flag

This release:

• improves performance of linting extremely large files with lots of lint errors by ~ 65%
• adds the --xhprof-dot foo.dot parameter to hhast-lint, to generate a .dot file suitable for use with GraphViz
• add --hhvm-4.2-to-4.3 to hhast-migrate as an alias of --ltgt-to-ne, replacing the <> operator with !=
• fixes a race condition in the LSP server that could lead to a crash; crashes are extremely harmful to performance with a JIT runtime
• added rewriteChildren() as a shallow alternative to rewriteDescendants()

Performance advice for linter authors

• rewriting the AST is expensive; avoid if (fixed_node($in) !== $in) { raise_error() }-like code
• prefer replace() over rewriteChildren() over rewriteDescendants() over rewrite()