Fix 7 stale references in docs and comments after codebase restructuring

- doc/DEVELOPING.md: src/main.cpp → src/tx_validation.cpp / block_processing.cpp
- doc/CHANGELOG-randomx-validation.md: src/main.cpp → src/block_processing.cpp (×2)
- doc/release-process.md: checkpoint instructions now reference
  src/chainparams_checkpoints_hush3.h instead of src/chainparams.cpp
- src/hush_defs.h: ./contrib/block_time.pl → ./contrib/scripts/block_time.pl (×3)
- src/cc/dapps/dappstd.c: commented hush_cJSON.c → hush_cJSON.cpp

doc/release-process.md

@@ -76,10 +76,10 @@ Install deps on Linux:
      - There is a hack in the script where you can hardcode a version number if hushd isn't running.
      - Comment out the HUSHVER line and uncomment the line above it with a hardcoded version number
    - PROTIP: Man page creation must be done after updating the version number and recompiling and before Debian package creation
- - Update checkpoints in src/chainparams.cpp via util/checkpoints.pl
+ - Update checkpoints in src/chainparams_checkpoints_hush3.h (or src/chainparams_checkpoints_dragonx.h for DRAGONX) via util/checkpoints.pl
    - Run "./util/checkpoints.pl help" to get example usage
    - hushd must be running to run this script, since it uses hush-cli to get the data
-   - Look for line which says "END HUSH mainnet checkpoint data" near line 560 in chainparams.cpp , that is where checkpoint data ends
+   - The checkpoint data is in src/chainparams_checkpoints_hush3.h, which is #included at line 559 of chainparams.cpp. Add new entries at the end of that header file.
    - Find the highest block height of checkpoint data, let's call it HEIGHT
    - Run `./util/checkpoints.pl 1000 HEIGHT &> checkpoints.txt` to generate the latest checkpoint data
    - To copy the new data from checkpoints.txt into the file, one way in Vim is to type ":r checkpoints.txt" which will read in a file and paste it as the current cursor