Tablet refactor

Summary:
Rename `Tablet` to `TabletReader`, corresponding to `TabletWriter` and split the 2 classes into their own files.

Also extract common constants and utils and leave them in `Tablet.h`.

EDIT:
After review, this diff now also includes:
* VeloxReader signature cleanup.
* Delete reader JNI related code.

Reviewed By: helfman

Differential Revision: D56445121

dwio/nimble/velox/TabletSections.h → dwio/nimble/tablet/Constants.h

@@ -17,6 +17,18 @@
 
 namespace facebook::nimble {
 
+constexpr uint16_t kMagicNumber = 0xA1FA;
+constexpr uint64_t kInitialFooterSize = 8 * 1024 * 1024; // 8Mb
+constexpr uint16_t kVersionMajor = 0;
+constexpr uint16_t kVersionMinor = 1;
+
+// Total size of the fields after the flatbuffer.
+constexpr uint32_t kPostscriptSize = 20;
+
+// The following fields in postscript are included in checksum calculation.
+// 4 bytes footer size + 1 byte compression type
+constexpr uint32_t kPostscriptChecksumedSize = 5;
+
 constexpr std::string_view kSchemaSection = "columnar.schema";
 constexpr std::string_view kMetadataSection = "columnar.metadata";