Multithreaded assemble_contigs.c (ctx_contigs.c)

Split out contig assembly code from
  src/commands/ctx_contigs -> src/tools/assemble_contigs.c

assemble_contigs.c: now multithreaded

hash_table.c: use random seed for initial hash seed, new seed for each htable
- this should ensure hash table has random order on each run

hash_table.h:
- HASH_ITERATE_PART loops until func() returns non-zero value