feat: ELECTRS_ROCKSDB_WRITE_BUFFER_SIZE

In testing setups large write buffer sizes lead to large
db files that potentially waste resources / interfere
with CIs limits.

This change adds an ability to overwrite the default behavior.

src/new_index/db.rs

@@ -80,6 +80,20 @@ pub enum DBFlush {
     Enable,
 }
 
+fn get_write_buffer_size() -> usize {
+    const VAR_NAME: &str = "ELECTRS_ROCKSDB_WRITE_BUFFER_SIZE";
+
+    if let Ok(var) = std::env::var(VAR_NAME) {
+        let size = FromStr::from_str(&var).expect("Could not parse {VAR_NAME}");
+
+        info!(size; "Using custom write buffer size");
+
+        size
+    } else {
+        256 << 20
+    }
+}
+
 impl DB {
     pub fn open(path: &Path, config: &Config) -> DB {
         debug!("opening DB at {:?}", path);
@@ -89,7 +103,7 @@ impl DB {
         db_opts.set_compaction_style(rocksdb::DBCompactionStyle::Level);
         db_opts.set_compression_type(rocksdb::DBCompressionType::Snappy);
         db_opts.set_target_file_size_base(1_073_741_824);
-        db_opts.set_write_buffer_size(256 << 20);
+        db_opts.set_write_buffer_size(get_write_buffer_size());
         db_opts.set_disable_auto_compactions(true); // for initial bulk load
 
         // db_opts.set_advise_random_on_open(???);