Change Context Pool Stripes from 4 to 128

Summary:
This diff increases the number of domains that locally cache a context from
4 to 128. In effect, this stops sharing contexts between cores, and instead
causes each core to have its own context. (If a machine has fewer than 128
CPUs, the unused slots will never be initialized.)

This diff is an experimental tuning. We don't expect it to have large positive
or negative effects, but we do currently see a non-zero amount of CPU spent
contending on the underlying pool's mutex. So this is an attempt to address
that. We'll look at CPU after it has rolled out and assess the results.

Reviewed By: terrelln

Differential Revision: D29944813

fbshipit-source-id: 7b10cb9dd2c49c02fad1f680a1bea02747990796

folly/compression/CompressionContextPoolSingletons.h

@@ -65,13 +65,13 @@ using ZSTD_CCtx_Pool = CompressionCoreLocalContextPool<
     ZSTD_CCtx_Creator,
     ZSTD_CCtx_Deleter,
     ZSTD_CCtx_Resetter,
-    4>;
+    128>;
 using ZSTD_DCtx_Pool = CompressionCoreLocalContextPool<
     ZSTD_DCtx,
     ZSTD_DCtx_Creator,
     ZSTD_DCtx_Deleter,
     ZSTD_DCtx_Resetter,
-    4>;
+    128>;
 
 /**
  * Returns a clean ZSTD_CCtx.