(Currently fails due to bug)

Check for unreasonable alloc size in LockedPool rather than lancing through new
Arenas until we improbably find one worthy of the quixotic request or the system
can support no more Arenas.

Use separate maps for used/free chunks to avoid linear scan through alloced
chunks for each alloc.

…geAllocator::AllocateLocked

This replaces the first-fit algorithm used in the Arena with a best-fit. According to "Dynamic Storage Allocation: A Survey and Critical Review", Wilson et. al. 1995, http://www.scs.stanford.edu/14wi-cs140/sched/readings/wilson.pdf, both startegies work well in practice.

The advantage of using best-fit is that we can switch the slow O(n) algorithm to O(log(n)) operations. Additionally, some previously O(log(n)) operations are now replaced with O(1) operations by using a hash map. The end effect is that the benchmark runs about 2.5 times faster on my machine:

old: BenchLockedPool, 5, 530, 5.25749, 0.00196938, 0.00199755, 0.00198172
new: BenchLockedPool, 5, 1300, 5.11313, 0.000781493, 0.000793314, 0.00078606

I've run all unit tests and benchmarks.

Changes in bitcoin#12048 cause a compilation error in Arena::walk() when
ARENA_DEBUG is defined. Specifically, Arena's chunks_free map was
changed to have a different value type.

Additionally, missing includes cause other compilation errors when
ARENA_DEBUG is defined.

Reproduced with:

make CPPFLAGS=-DARENA_DEBUG

The test uses reinterpret_cast<void*> on unallocated memory. Using this
memory in printchunk as char* causes a segfault, so have printchunk take
void* instead.