Mark Set{Items,Bytes}Processed()/{items,bytes}_processed() as depreca…

…ted. (#654)

They are basically proto-version of custom user counters.
It does not seem that they do anything that custom user counters
don't do. And having two similar entities is not good for generalization.

Migration plan:
* ```
  SetItemsProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate)},
    ...
  });
  ```
* ```
  SetBytesProcessed(<val>)
    =>
  state.counters.insert({
    {"<Name>", benchmark::Counter(<val>, benchmark::Counter::kIsRate,
                                  benchmark::Counter::OneK::kIs1024)},
    ...
  });
  ```
* ```
  <Name>_processed()
    =>
  state.counters["<Name>"]
  ```

One thing the custom user counters miss is better support
for units of measurement.

Refs. #627

include/benchmark/benchmark.h

@@ -542,9 +542,17 @@ class State {
   //
   // REQUIRES: a benchmark has exited its benchmarking loop.
   BENCHMARK_ALWAYS_INLINE
+  BENCHMARK_DEPRECATED_MSG(
+      "The SetItemsProcessed()/items_processed()/SetBytesProcessed()/"
+      "bytes_processed() will be removed in a future release. "
+      "Please use custom user counters.")
   void SetBytesProcessed(int64_t bytes) { bytes_processed_ = bytes; }
 
   BENCHMARK_ALWAYS_INLINE
+  BENCHMARK_DEPRECATED_MSG(
+      "The SetItemsProcessed()/items_processed()/SetBytesProcessed()/"
+      "bytes_processed() will be removed in a future release. "
+      "Please use custom user counters.")
   int64_t bytes_processed() const { return bytes_processed_; }
 
   // If this routine is called with complexity_n > 0 and complexity report is
@@ -565,9 +573,17 @@ class State {
   //
   // REQUIRES: a benchmark has exited its benchmarking loop.
   BENCHMARK_ALWAYS_INLINE
+  BENCHMARK_DEPRECATED_MSG(
+      "The SetItemsProcessed()/items_processed()/SetBytesProcessed()/"
+      "bytes_processed() will be removed in a future release. "
+      "Please use custom user counters.")
   void SetItemsProcessed(int64_t items) { items_processed_ = items; }
 
   BENCHMARK_ALWAYS_INLINE
+  BENCHMARK_DEPRECATED_MSG(
+      "The SetItemsProcessed()/items_processed()/SetBytesProcessed()/"
+      "bytes_processed() will be removed in a future release. "
+      "Please use custom user counters.")
   int64_t items_processed() const { return items_processed_; }
 
   // If this routine is called, the specified label is printed at the