TS-1764: fix spdy plugin build
TS-1771: add http_load to the build
TS-1766: integrate AIO test into the autotools test suite

  - Add some const to Diags tags API.
  - Use _SOURCES rather than _LDADD to build source files.
  - Remove unused macro MGMT_PTR.
  - Remove ProcessManager usage of global_config_cbs global to
    reduce the number of global dependencies.
  - Add AIO unit test to the build.
  - Move AIO tests into a single file.
Update CHANGES for TS-1771