os/bluestore: remove unneeded indirection in BitMapZone.

os/bluestore: remove unneeded indirection in BitMapZone.

BitMapZone aggregates BmapEntries through a pointer to independently-
allocated std::vector which in turn has its own pointer to the real
data. This indirection is unnecessary as std::vector is pretty cheap
in the terms of memory overhead. For instance, on x86-64 & GCC 5.4.0
sizeof(std::vector) == 24, which translates to 3 raw pointers.

Stripping the indirection could have positive impact on processor's
caches and allow to minimize the number of memory accesses.

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