ctdb-ipalloc: Optimise check to see if IP is available on a node

ctdb-ipalloc: Optimise check to see if IP is available on a node

Use a "bitmap" of available IPs for each IP address instead of walking
the list of available IP addresses.

For ctdb/tests/takeover/lcp2.030.sh, this improves the time taken on
my laptop from:

  real	0m11.997s
  user	0m11.960s
  sys	0m0.000s

to

  real	0m8.571s
  user	0m8.544s
  sys	0m0.000s

So, when assigning all 900 IP addresses the improvement is about 25%.

For the no-op case (where all IPs are already assigned to nodes), the
extra setup adds a small fraction of a second for 900 IPs.
Intermediate cases result in intermediate improvements.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>