refactor: Extract default batch size into kernel

The constant for the default batch size is moved to `kernel/caches.h` to consolidate kernel-related cache constants.

src/txdb.h

@@ -8,6 +8,7 @@
 
 #include <coins.h>
 #include <dbwrapper.h>
+#include <kernel/caches.h>
 #include <kernel/cs_main.h>
 #include <sync.h>
 #include <util/fs.h>
@@ -21,16 +22,12 @@
 class COutPoint;
 class uint256;
 
-//! -dbbatchsize default (bytes)
-static const int64_t nDefaultDbBatchSize = 16 << 20;
-
 //! User-controlled performance and debug options.
 struct CoinsViewOptions {
     //! Maximum database write batch size in bytes.
-    size_t batch_write_bytes = nDefaultDbBatchSize;
-    //! If non-zero, randomly exit when the database is flushed with (1/ratio)
-    //! probability.
-    int simulate_crash_ratio = 0;
+    size_t batch_write_bytes{DEFAULT_DB_CACHE_BATCH};
+    //! If non-zero, randomly exit when the database is flushed with (1/ratio) probability.
+    int simulate_crash_ratio{0};
 };
 
 /** CCoinsView backed by the coin database (chainstate/) */