feat: increase default and max FVM concurrency

We used to allocate 1024 instances per "concurrency" but we now only
allocate 1024 plus 20 per "concurrency". So, I'm increasing the default
and maximum concurrency factors to take advantage of this. Importantly:

1. Max concurrency 128 -> 4096. `1024 + 4096*20 < 128 * 1024`.
2. Default concurrency 4 -> 150. `1024 + 150*20 < 4 * 1024`.

Motivation: I'm hoping that the default (150) is sufficient (more than
the previous max...) so we can just get rid of this knob.

fixes filecoin-project/lotus#11817

rust/src/fvm/engine.rs

@@ -66,7 +66,7 @@ pub struct MultiEngineContainer {
 }
 
 const LOTUS_FVM_CONCURRENCY_ENV_NAME: &str = "LOTUS_FVM_CONCURRENCY";
-const VALID_CONCURRENCY_RANGE: RangeInclusive<u32> = 1..=128;
+const VALID_CONCURRENCY_RANGE: RangeInclusive<u32> = 1..=4096;
 
 impl TryFrom<u32> for EngineVersion {
     type Error = anyhow::Error;
@@ -81,9 +81,9 @@ impl TryFrom<u32> for EngineVersion {
 }
 
 impl MultiEngineContainer {
-    /// Constructs a new multi-engine container with the default concurrency (4).
+    /// Constructs a new multi-engine container with the default concurrency (150).
     pub fn new() -> MultiEngineContainer {
-        Self::with_concurrency(4)
+        Self::with_concurrency(150)
     }
 
     /// Constructs a new multi-engine container with the concurrency specified in the