pageserver: add separate, disabled compaction semaphore (#10716)

## Problem

L0 compaction can get starved by other background tasks. It needs to be
responsive to avoid read amp blowing up during heavy write workloads.

Touches #10694.

## Summary of changes

Add a separate semaphore for compaction, configurable via
`use_compaction_semaphore` (disabled by default). This is primarily for
testing in staging; it needs further work (in particular to split
image/L0 compaction jobs) before it can be enabled.

libs/pageserver_api/src/config.rs

@@ -94,6 +94,7 @@ pub struct ConfigToml {
     pub ondemand_download_behavior_treat_error_as_warn: bool,
     #[serde(with = "humantime_serde")]
     pub background_task_maximum_delay: Duration,
+    pub use_compaction_semaphore: bool,
     pub control_plane_api: Option<reqwest::Url>,
     pub control_plane_api_token: Option<String>,
     pub control_plane_emergency_mode: bool,
@@ -470,6 +471,7 @@ impl Default for ConfigToml {
                 DEFAULT_BACKGROUND_TASK_MAXIMUM_DELAY,
             )
             .unwrap()),
+            use_compaction_semaphore: false,
 
             control_plane_api: (None),
             control_plane_api_token: (None),