OAK-12325: Use heap based comparison of blob content

[nfsantos]

Jira: https://issues.apache.org/jira/browse/OAK-12325

Problem

AbstractBlob.equal()'s byte-wise fallback comparison (IOUtils.contentEquals(InputStream, InputStream)) allocates two direct ByteBuffers per call when the underlying streams are file-backed (e.g. FileChannel-based NIO copies). This comparison runs on hot paths such as the segment writer's dedup check when writing an unchanged property and node-state diffing (rebase, commit diff). Under write-heavy load this becomes a significant source of native (off-heap) memory allocation — direct buffers are reclaimed only via GC-driven Cleaner/PhantomReference callbacks, which can lag behind allocation and drive up process RSS outside the JVM heap.

Summary

• Replace IOUtils.contentEquals in AbstractBlob.equal() with a manual buffered comparison using InputStream.readNBytes and Arrays.equals, working entirely with heap byte[] buffers that are reclaimed by ordinary GC, avoiding the commons-io dependency on this hot path.
• Also close both input streams via try-with-resources, fixing a related stream leak in the previous implementation.