job: increase JOB_BUFFER_SIZE to 0xFFFF

It used to be 1024 bytes, which is very tiny and slows down some operations
(imaging `cat`-ing a large file). Benchmarks show a large speedup for such
cases. ref #978.

For modern systems 0xFFFF bytes (65535 B = 64 KB = 0.0625 MB) per job
shouldn't be a big problem.

src/nvim/os/job.c

@@ -21,7 +21,7 @@
 
 #define EXIT_TIMEOUT 25
 #define MAX_RUNNING_JOBS 100
-#define JOB_BUFFER_SIZE 1024
+#define JOB_BUFFER_SIZE 0xFFFF
 
 struct job {
   // Job id the index in the job table plus one.