SLING-2707 Support of chunked file upload into Sling

Patch with feature details at
https://cwiki.apache.org/confluence/display/SLING/Chunked+File+Upload+Support
Salient features:
1. Extension to SlingPostServlet
2. Chunk upload is supported in "modify"operation of SlingPostServlet
( ie. the default operation) . No new operation introduced for it.
3. Chunks are stored within actual path in mixin node type.
Defined [sling:chunk] node type and [sling:chunks] mixin for this purpose.
4. Added integration test cases validating different scenarios of
chunk upload.
SlingPostChunkUploadTest#testChunkUpload"
SlingPostChunkUploadTest#testInterruptedChunkUpload
SlingPostChunkUploadTest#testConcurrentChunkUpload
SlingPostChunkUploadTest#testMidwayChunkUpload
SlingPostChunkUploadTest#testChunkUploadOnExistingNode
SlingPostChunkUploadTest#testChunkUploadOnStreaming