opt(Restore): Make restore map phase faster (#8038)

With this change, we can get ~450 MBps output throughput for map phase sustained. This is what we got on a 48 core AWS machine.

```
alpha1    | I0917 08:03:27.921912      17 restore_map.go:547] Restore MAP 01h04m30s len(reqCh): 0 len(writeCh): 0 read: 472 GiB. output: 1.6 TiB. rate: 437 MiB/sec. nextFileId: 2474 writers: 0 jemalloc: 0 B.
alpha1    | I0917 08:03:27.921934      17 restore_map.go:559] Restore MAP Done in 01h04m30s.
```

Changes:
* Make numGo equal to number of cores
* Don't throttle.
* Create a bigger buffer before calling merge
* Rewrite mapper to remove sendForWriting out of the critical path.
* Fix up a deadlock
* Fix up the mapper
* add write ch
* Use many goroutines to create map files
* Use fewer goroutines
* Use 75% of the cores for mapping
* Stagger the writes
* Half writers can write at a time
* Print num writers
* Range for file would be 1/4 to 1
* Reduce number of pending requests

Co-authored-by: Ahsan Barkati <[email protected]>