Fix issues found with `sequence`
Fix issues found with `sequence`

1. Call `HandleCommand()` without the context argument. This is because the
context is essentially a cache, and its properties can be invalided by previous
commands. For example if one command is `finish` then the `frame` property of
the context will now be invalid and unusable. `HandleCommand()` without the
context argument will use the current frame, for example.
2. Set the debugger to synchronous execution. Each command in the sequence must
not run until the previous command has finished. If the debugger is in async
mode, `HandleCommand()` can return asynchronously before the command has
concluded.
3. Special case `continue` by calling directly `process.Continue()`. This is
done as a workaround to `HandleCommand('continue')` not resulting in the
expected result, it's as though the continue command is interrupted. Calling
`Continue()` does result in the expected behavior.

facebook/chisel,facebook/chisel,facebook/chisel