[SDK] Fix double buffer data race (#394)

* Fix double buffer data race

- fixes relatively rare data race in double buffering scheme

In `rocprofiler::buffer::instance::emplace`, the `container::record_header_buffer::get_record_headers()` function returned a `std::vector<rocprofiler_record_header_t*>` and then invoked callback to tool. It was possible for that callback to still be executing while the buffer was being updated. This potentially introduced a scenario where the rocprofiler_record_header_t* was modified (or corrupted) before the tool processed the record. In rocprofv3, this would result in a "future" buffer record showing up among "past" buffer records. E.g., correlation id sequence of 1-15 where the buffer flushes after five values, could result in this during processing:
|     |     |     |     |      |
|:---:|:---:|:---:|:---:|:---:|
|  1 |  2 |  3 |  4 | 15 |
|  6 |  7 |  8 |  9 | 10 |
| 11 | 12 | 13 | 14 | 15 |

Because buffer A (of double buffering scheme) originally containing corr ids 1-5 stalled after process corr id 4 (e.g. write to disk), buffer B filled up with 6-10 and started flushing, causing a switch back to buffer A, and buffer A was filled with 11-15 by the time callback accessed what was originally corr id 5 but was now updated to corr id 15.

* Update CHANGELOG

* misc minor cleanup

---------

Co-authored-by: Jonathan R. Madsen <jonathanrmadsen@gmail.com>

source/lib/rocprofiler-sdk/tests/buffer.cpp

@@ -55,10 +55,7 @@ TEST(rocprofiler_lib, buffer)
     EXPECT_EQ(buffer_v->buffer_id, buffer_id->handle);
 
     buffer_v->watermark = common::units::get_page_size();
-    {
-        auto records = buffer_v->get_internal_buffer().get_record_headers();
-        EXPECT_EQ(records.size(), 0);
-    }
+    EXPECT_EQ(buffer_v->get_internal_buffer().get_num_record_headers(), 0);
 
     EXPECT_TRUE(buffer_v->get_internal_buffer().allocate(sizeof(rocprofiler_buffer_id_t)));
 
@@ -67,8 +64,7 @@ TEST(rocprofiler_lib, buffer)
     auto data = *buffer_id;
     buffer_v->emplace(1, 1, data);
 
-    auto records = buffer_v->get_internal_buffer().get_record_headers();
-    EXPECT_EQ(records.size(), 1);
+    EXPECT_EQ(buffer_v->get_internal_buffer().get_num_record_headers(), 1);
 
     auto flush_status = buffer::flush(*buffer_id, true);
     EXPECT_EQ(flush_status, ROCPROFILER_STATUS_SUCCESS);