bluestore: remove CephContext* from BmapEntry.

bluestore: remove CephContext* from BmapEntry.

Each BmapEntry instance stores a pointer to the same CephContext.
As we expect to have thousands of instances the overhead might
be too high. For instance, serving 1 TiB SSD disk on x86-64,
while using the default settings, results in 32 MiB of extra
memory consumption:

  # assuming sizeof(unsigned long) * CHAR_BIT == 64
  >>> 1024 * 1024 * 1024 * 1024 / 4096 / 64
  4194304
  >>> 4194304 * 8 / 1024
  32768

Although memory is cheap, CPU's caches are not.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>