Add fixed-size, preallocated pool support

Add fixed-size, preallocated pool support

Simply call pool_init_fixed(...) before using the pool. Note that
fixed, preallocated pools live in individually-mmap'ed address
segments, except for the free element bitmap. A large fixed pool can
exceed 4gb.

Fix tcp buffer allocator leak, remove broken assert

Change-Id: I4421082e12a77c41c6e20f7747f3150dcd01fc26
Signed-off-by: Dave Barach <dave@barachs.net>