Expose a generic read method that can be extended to have more contro…

…l over how the `readFn` is called, for example concurrency limits or combination of requests that are close in offsets

lib/reader.js

@@ -289,11 +289,15 @@ class ParquetEnvelopeReader {
   }
 
   constructor(readFn, closeFn, fileSize) {
-    this.read = readFn;
+    this.readFn = readFn;
     this.close = closeFn;
     this.fileSize = fileSize;
   }
 
+  async read(offset, length) {
+    return this.readFn(offset, length);
+  }
+
   async readHeader() {
     let buf = await this.read(0, PARQUET_MAGIC.length);