Refactoring: Introduce extensibility scheme for tracepoint structures

- Introduce struct_size extensibility scheme for all tracepoint public
  structures, except for struct lttng_ust_tracepoint_probe which is
  embedded in a public array of structures, thus making it tricky for
  instrumentation sites to efficiently skip over the array elements
  without reading their size.
- Remove padding from those structures.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I2387a62c03f6f729c59ddc3fafa3a9b5546fb287