OPTIM: startup: fast unique_id allocation for acl.

OPTIM: startup: fast unique_id allocation for acl.

pattern_finalize_config() uses an inefficient algorithm which is a
problem with very large configuration files. This affects startup, and
therefore reload time. When haproxy is deployed as a router in a
Kubernetes cluster the generated configuration file may be large and
reloads are frequently occuring, which makes this a significant issue.

The old algorithm is O(n^2)
* allocate missing uids - O(n^2)
* sort linked list - O(n^2)

The new algorithm is O(n log n):
* find the user allocated uids - O(n)
* store them for efficient lookup - O(n log n)
* allocate missing uids - n times O(log n)
* sort all uids - O(n log n)
* convert back to linked list - O(n)

Performance examples, startup time in seconds:

    pat_refs old     new
    1000      0.02   0.01
    10000     2.1    0.04
    20000    12.3    0.07
    30000    27.9    0.10
    40000    52.5    0.14
    50000    77.5    0.17

Please backport to 1.8, 2.0 and 2.1.

(cherry picked from commit f91ac19299fe216a793ba6550dca06b688b31549)
Signed-off-by: Willy Tarreau <w@1wt.eu>