index-pack: support multithreaded delta resolving

This puts delta resolving on each base on a separate thread, one base
cache per thread. Per-thread data is grouped in struct thread_local.
When running with nr_threads == 1, no pthreads calls are made. The
system essentially runs in non-thread mode.

An experiment on a Xeon 24 core machine with git.git shows that
performance does not increase proportional to the number of cores. So
by default, we use maximum 3 cores. Some numbers with --threads from 1
to 16:

1..4
real    0m8.003s  0m5.307s  0m4.321s  0m3.830s
user    0m7.720s  0m8.009s  0m8.133s  0m8.305s
sys     0m0.224s  0m0.372s  0m0.360s  0m0.360s

5..8
real    0m3.727s  0m3.604s  0m3.332s  0m3.369s
user    0m9.361s  0m9.817s  0m9.525s  0m9.769s
sys     0m0.584s  0m0.624s  0m0.540s  0m0.560s

9..12
real    0m3.036s  0m3.139s  0m3.177s  0m2.961s
user    0m8.977s  0m10.205s 0m9.737s  0m10.073s
sys     0m0.596s  0m0.680s  0m0.684s  0m0.680s

13..16
real    0m2.985s  0m2.894s  0m2.975s  0m2.971s
user    0m9.825s  0m10.573s 0m10.833s 0m11.361s
sys     0m0.788s  0m0.732s  0m0.904s  0m1.016s

On an Intel dual core and linux-2.6.git

1..4
real    2m37.789s 2m7.963s  2m0.920s  1m58.213s
user    2m28.415s 2m52.325s 2m50.176s 2m41.187s
sys     0m7.808s  0m11.181s 0m11.224s 0m10.731s

Thanks Ramsay Jones for troubleshooting and support on MinGW platform.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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