[scudo][standalone] Release secondary memory on purge

Summary:
The Secondary's cache needs to be released when the Combined's
`releaseToOS` function is called (via `M_PURGE`) for example,
which this CL adds.

Additionally, if doing a forced release, we'll release the
transfer batch class as well since now we can do that.

There is a couple of other house keeping changes as well:
- read the page size only once in the Secondary Cache `store`
- remove the interval check for `CanRelease`: we are going to
  make that configurable via `mallopt` so this needs not be
  set in stone there.

Reviewers: cferris, hctim, pcc, eugenis

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73730