Refactoring: context structures

The context structures are now hidden in headers which are not
installed, but those are still part of the UST ABI. Therefore, we need
to update them to make sure those are extensible.

- Namespace theses structures with lttng_ust_ prefix,
- Use struct_size extensibility scheme. There was no prior padding, so
  extensibility was an issue in the past.
- Remove padding.
- This requires that none of those structures can be nested. It also
  requires that all arrays of structures must be replaced by arrays of
  pointers to structures.
- Replace struct lttng_perf_counter_field pointer by a private data
  pointer.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I4ecb8f73652770f15c693c1b5b2acce3e871a64b