page-read: Implement asynchronous mode of reading pages (v2)

Currently each page read request results in pread() call. But
we can do better -- if there's no urgent need in data (which is
likely the case for non-cow pagemaps) we can save where the
date should go to and then read it in in one preadv syscall.

For lazy pages we get an ability to do async read_pages call
that would send the request to server and return w/o data in
the buffer.

The explicit sync() method is added since .close is void and
nobody checks its return code.

v2: Lost limits.h in pagemap.c for IOV_MAX (compilation).
v3: split added code into smaller chunks

https://travis-ci.org/criupatchwork/criu/builds/175451746


Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>