introduced parameter ZSTD_p_nonBlockingMode

This new parameter makes it possible to call
streaming ZSTDMT with a single thread set
which is non blocking.

It makes it possible for the main thread to do other tasks in parallel
while the worker thread does compression.
Typically, for zstd cli, it means it can do I/O stuff.

Applied within fileio.c, this patch provides non-negligible gains during compression.

Tested on my laptop, with enwik9 (1000000000 bytes) : time zstd -f enwik9

With traditional single-thread blocking mode :
real    0m9.557s
user    0m8.861s
sys     0m0.538s

With new single-worker non blocking mode :
real    0m7.938s
user    0m8.049s
sys     0m0.514s

=> 20% faster

lib/zstd.h

@@ -972,10 +972,20 @@ typedef enum {
     ZSTD_p_dictIDFlag,       /* When applicable, dictionary's ID is written into frame header (default:1) */
 
     /* multi-threading parameters */
+    /* These parameters are only useful if multi-threading is enabled (ZSTD_MULTITHREAD).
+     * They return an error otherwise. */
     ZSTD_p_nbThreads=400,    /* Select how many threads a compression job can spawn (default:1)
                               * More threads improve speed, but also increase memory usage.
                               * Can only receive a value > 1 if ZSTD_MULTITHREAD is enabled.
                               * Special: value 0 means "do not change nbThreads" */
+    ZSTD_p_nonBlockingMode,  /* Single thread mode is by default "blocking" :
+                              * it finishes its job as much as possible, and only then gives back control to caller.
+                              * In contrast, multi-thread is by default "non-blocking" :
+                              * it takes some input, flush some output if available, and immediately gives back control to caller.
+                              * Compression work is performed in parallel, within worker threads.
+                              * (note : a strong exception to this rule is when first job is called with ZSTD_e_end : it becomes blocking)
+                              * Setting this parameter to 1 will enforce non-blocking mode even when only 1 thread is selected.
+                              * It allows the caller to do other tasks while the worker thread compresses in parallel. */
     ZSTD_p_jobSize,          /* Size of a compression job. This value is only enforced in streaming (non-blocking) mode.
                               * Each compression job is completed in parallel, so indirectly controls the nb of active threads.
                               * 0 means default, which is dynamically determined based on compression parameters.