blk_cache: fix deadlock in allocator hierarchy

This commit generalizes the bit array in 'base/util/bit_array.h',
so that it can be used in a statically, when the array size is known
at compile time, or dynamically. It uses the dynamic approach of the
bit array for a more generalized version of the packet allocator,
formerly only used by NIC session clients. The more generic packet
allocator is used by the block cache to circumvent the allocation
deadlock described in issue #1059.

Fixes #1059