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
Change the template parameter for Bit_allocator, and Bit_array. Instead of
assigning words to be used by the bit array, you can now tell the count of
items that shall be used.
Moreover, some dead code, previously using the Bit_allocator, was removed.
Related to #1024
