Drop experimental_multi_threaded_digest

[meisterT]

This options defaults to true since a while and should just be removed, likely with the whole SsdModule.

[bjacklyn]

@meisterT -- checking cached actions now takes 5-6x longer in my local builds after 6ac6954224b2b74c18d3218dfa299424cbc944fb was merged, that is what used to take 30 seconds for bazel to determine there was nothing to do in an already fully cached build, it now takes 2 mins 30 seconds

[bjacklyn]

There used to be a check against this boolean which I assume was defaulting to true:

-  private static final AtomicBoolean MULTI_THREADED_DIGEST = new AtomicBoolean(false);
...
-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE && !MULTI_THREADED_DIGEST.get()) {
+    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
       // We'll have to read file content in order to calculate the digest.
       // We avoid overlapping this process for multiple large files, as
       // seeking back and forth between them will result in an overall loss of
       // throughput.
       digest = getDigestInExclusiveMode(path);
     } else {
-      digest = getDigestInternal(path);
+      digest = path.getDigest();
     }

But now it always takes the slow path. I revived the fast checking cached actions with this:

-    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE) {
+    if (fileSize > MULTI_THREADED_DIGEST_MAX_FILE_SIZE && !true) {

[meisterT]

Ouch, that's definitely a mistake. I'm OOO this week but feel free to send a PR to fix the behavior.

cc @Wyverald this is a release blocker

[Wyverald]

Cherrypicks are still open.

[brentleyjones]

Should be reopened as it's still blocking 5.0 (needs to be cherry-picked).

[Wyverald]

Cherrypicked.