LogRecord and SpanByte changes

[TedHartMS]

This is an initial draft of a PR to convert Garnet to use a new ObjectAllocator with a revised on-disk format that will use only a single log file, change ISessionFunctions to be (Disk)LogRecord-based and propagate this to operations such as Compaction and Migration, remove TKey and TValue from TsavoriteKV, and many related refactors.

This PR will be long-lived as additional task PRs are merged into storage-v2 before it is ready for merge to main (see "Major Remaining Tasks" below). Currently this branch implements ONLY the in-memory portion; the IO work is not yet implemented.

Highlights:

• ArgSlice and SpanByte have been merged to a PinnedSpanByte struct with much more limited use. SpanByte remains as a static class providing (ReadOnly)Span<byte> extensions.
• All keys are now either PinnedSpanByte (mostly at and above the GarnetApi layer) or ReadOnlySpan<byte> at the StorageSession and below, including Tsavorite. There are no longer any byte[] keys. The TKey type argument is gone from Tsavorite and Garnet.
  • Non-SpanByte KeyComparer implementations have been removed.
• Values are either (ReadOnly)Span<byte> or objects implementing IHeapObject. TValue has been removed from TsavoriteKV.
• "ref TKey", "ref TValue", and "ref RecordInfo" on ISessionFunctions methods are replaced by one of the LogRecord variants, where they are properties.
  • See https://github.com/microsoft/garnet/tree/tedhar/object-allocator/website/docs/dev/tsavorite/logrecord.md for details of LogRecord and associated classes and structs.
• ETag and Expiration are now properties on LogRecord rather than embedded within Value or IGarnetObject. Internally, they are after the Value and *LogRecord manages shifting as Value shrinks and grows; all "record extra length" management is done by LogRecord and is opaque to ISessionFunctions.
• GenericAllocator has been replaced by ObjectAllocator; for details, see https://github.com/microsoft/garnet/tree/tedhar/object-allocator/website/docs/dev/tsavorite/logrecord.md. Currently only in-memory operations are implemented.
• For ObjectAllocator, keys and string values that are past a configurable size can "overflow" to out-of-line byte[], keeping the record size manageable. Tsavorite implements this threshold automatically, and LogRecord manages it internally when returning the Key or ValueSpan.
• SpanByteScanIterator and GenericScanIterator have been unified into RecordScanIterator
• Some names have been changed for clarity:
  • "Lockable*" and "Manual*" have become "Transactional*"
  • "Transient" locking has become "Ephemeral"

Major remaining tasks (this will be updated as they are implemented):

• Complete ObjectAllocator single-LogFile IO.
• Change object-size tracking from page-level granularity to record-level.
• Performance evaluation and tuning
• Fix RENAME and any other remaining areas where the Expiration is not propagated as before, as it is no longer part of Value (including IGarnetObject)
• Clean up error handling related to the VarLenInputMethods functions (make them consistent with returns from RMWMethods, etc.)
• Implement JsonObject MemorySize/DiskSize
• Finish converting and re-enabling all Tsavorite Unit Tests
• Revise the current Garnet.common to be Garnet.core, then re-create Garnet.common as a common layer between Tsavorite and the Garnet processing layer, containing PinnedSpanByte, epochs, Utility functions (some of which are currently duplicated), and so on.
• Investigate the performance impact of moving further along the (ReadOnly)Span<byte> path by replacing byte* usage (such as was done for Utility.HashBytes).