ethdb/pebble: adjust the number of memory tables (ethereum#31970)

This pull request adjusts the number of allowed memory tables in Pebble.

Pebble allows configuring an arbitrary number of memory tables to hold 
unflushed data. When the current memtable becomes full, it is scheduled 
for flushing, and a new memtable is allocated to accept subsequent
writes. However, if too many memtables accumulate and are waiting to be 
flushed, subsequent writes will stall.

Originally, only two memtables were configured, each with a size of 512
MB for Ethereum mainnet. While this setup works well under normal
conditions, it becomes problematic under heavy write loads. In such scenarios,
flushing is only triggered when more than 512 MB of data is pending, which may
not be responsive enough. Even worse, if compactions are running
concurrently, flushing memtables can become slow due to the heavy IO 
overhead, leading to write stalls across the system.

This pull request tries to mitigate the performance degradation by having 
more memory tables but with a smaller size. In this case, the pending
writes can be flushed more smoothly and responsively.

Author: rjl493456442

ethdb/pebble/pebble.go

@@ -199,9 +199,12 @@ func New(file string, cache int, handles int, namespace string, readonly bool) (
 	// Taken from https://github.com/cockroachdb/pebble/blob/master/internal/constants/constants.go
 	maxMemTableSize := (1<<31)<<(^uint(0)>>63) - 1
 
-	// Two memory tables is configured which is identical to leveldb,
-	// including a frozen memory table and another live one.
-	memTableLimit := 2
+	// Four memory tables are configured, each with a default size of 256 MB.
+	// Having multiple smaller memory tables while keeping the total memory
+	// limit unchanged allows writes to be flushed more smoothly. This helps
+	// avoid compaction spikes and mitigates write stalls caused by heavy
+	// compaction workloads.
+	memTableLimit := 4
 	memTableSize := cache * 1024 * 1024 / 2 / memTableLimit
 
 	// The memory table size is currently capped at maxMemTableSize-1 due to a