loadblk() calls are potentially slow and external code that serve the cal can
take other locks in addition to virtmem lock taken by virtmem subsystem. If
that "other locks" are also taken before external code calls e.g.
fileh_invalidate_page() in different codepath a deadlock can happen, e.g.

      T1                  T2

      page-access         invalidation-from-server received
      V -> loadblk
                          Z   <- ClientStorage.invalidateTransaction()
      Z -> zeo.load
                          V   <- fileh_invalidate_page

The solution to avoid deadlock is to call loadblk() with virtmem lock released
and upon loadblk() completion recheck virtmem data structures carefully.

To make that happen:

- new page state is introduces:

    PAGE_LOADING                (file content loading is  in progress)

- virtmem releases virt_lock before calling loadblk() when serving pagefault

- because loading is now done with virtmem lock released, now:

1. After loading completes we need to recheck fileh/vma data structures

   The recheck is done in full - vma_on_pagefault() just asks its driver (see
   VM_RETRY and VM_HANDLED codes) to retry handling the fault completely. This
   should work as the freshly loaded page was just inserted into fileh->pagemap
   and should be found there in the cache on next lookup.

   On the other hand this also works correctly, if there was concurrent change
   - e.g. vma was unmapped while we were loading the data - in that case the
   fault will be also processed correctly - but loaded data will stay in
   fileh->pagemap (and if not used will be evicted as not-needed
   eventually by RAM reclaim).

2. Similar to retrying mechanism is used for cases when two threads
   concurrently access the same page and would both try to load corresponding
   block - only one thread issues the actual loadblk() and another waits for load
   to complete with polling and VM_RETRY.

3. To correctly invalidate loading-in-progress pages another new page state
   is introduced:

    PAGE_LOADING_INVALIDATED    (file content loading was in progress
                                 while request to invalidate the page came in)

   which fileh_invalidate_page() uses to propagate invalidation message to
   loadblk() caller.

4. Blocks loading can now happen in parallel with other block loading and
   other virtmem operations - e.g. invalidation. For such cases tests are added
   to test_thread.py

5. virtmem lock now becomes just regular lock, instead of being previously
   recursive.

   For virtmem lock to be recursive was needed for cases, when code under
   loadblk() could trigger other virtmem calls, e.g. due to GC and calling
   another VMA dtor that would want to lock virtmem, but virtmem lock was
   already held.

   This is no longer needed.

6. To catch double faults we now cannot use just on static variable
   in_on_pagefault. That variable thus becomes thread-local.

7. Old test in test_thread to "test that access vs access don't overlap" no
   longer holds true - and is thus removed.

/cc @Tyagov, @klaus