Handle transaction global pool allocation centrally in rpmtsPool()

Handle transaction global pool allocation centrally in rpmtsPool()

- Previously the pool would only get allocated on successfull addition
  of install or erasure elements, causing assert() failures on
  operations on empty transaction set, which should be just a no-op,
  not an error.
- Make rpmtsPool() create the pool if it doesn't exist, update relevant
  users to call rpmtsPool() instead of directly accessing tsmem->pool,
  this avoids having to worry about pool existence in all the various cases.
- Also fix up the pool-related comment on rpmtsEmpty(): pools does not
  and can not support emptying as it could break references to its
  contents. Per-string refcount would be needed for emptying support.
(cherry picked from commit d73535e1a9dc5095e78475adc5b636d99f01efa9)