parallel-checkout: make it truly parallel

Use multiple worker processes to distribute the queued entries and call
write_pc_item() in parallel for them. The items are distributed
uniformly in contiguous chunks. This minimizes the chances of two
workers writing to the same directory simultaneously, which could affect
performance due to lock contention in the kernel. Work stealing (or any
other format of re-distribution) is not implemented yet.

The protocol between the main process and the workers is quite simple.
They exchange binary messages packed in pkt-line format, and use
PKT-FLUSH to mark the end of input (from both sides). The main process
starts the communication by sending N pkt-lines, each corresponding to
an item that needs to be written. These packets contain all the
necessary information to load, smudge, and write the blob associated
with each item. Then it waits for the worker to send back N pkt-lines
containing the results for each item. The resulting packet must contain:
the identification number of the item that it refers to, the status of
the operation, and the lstat() data gathered after writing the file (iff
the operation was successful).

For now, checkout always uses a hardcoded value of 2 workers, only to
demonstrate that the parallel checkout framework correctly divides and
writes the queued entries. The next patch will add user configurations
and define a more reasonable default, based on tests with the said
settings.

Co-authored-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Co-authored-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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