[SPARK-9451] [SQL] Support entries larger than default page size in BytesToBytesMap & integrate with ShuffleMemoryManager

This patch adds support for entries larger than the default page size in BytesToBytesMap.  These large rows are handled by allocating special overflow pages to hold individual entries.

In addition, this patch integrates BytesToBytesMap with the ShuffleMemoryManager:

- Move BytesToBytesMap from `unsafe` to `core` so that it can import `ShuffleMemoryManager`.
- Before allocating new data pages, ask the ShuffleMemoryManager to reserve the memory:
  - `putNewKey()` now returns a boolean to indicate whether the insert succeeded or failed due to a lack of memory.  The caller can use this value to respond to the memory pressure (e.g. by spilling).
- `UnsafeFixedWidthAggregationMap. getAggregationBuffer()` now returns `null` to signal failure due to a lack of memory.
- Updated all uses of these classes to handle these error conditions.
- Added new tests for allocating large records and for allocations which fail due to memory pressure.
- Extended the `afterAll()` test teardown methods to detect ShuffleMemoryManager leaks.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7762 from JoshRosen/large-rows and squashes the following commits:

ae7bc56 [Josh Rosen] Fix compilation
82fc657 [Josh Rosen] Merge remote-tracking branch 'origin/master' into large-rows
34ab943 [Josh Rosen] Remove semi
31a525a [Josh Rosen] Integrate BytesToBytesMap with ShuffleMemoryManager.
626b33c [Josh Rosen] Move code to sql/core and spark/core packages so that ShuffleMemoryManager can be integrated
ec4484c [Josh Rosen] Move BytesToBytesMap from unsafe package to core.
642ed69 [Josh Rosen] Rename size to numElements
bea1152 [Josh Rosen] Add basic test.
2cd3570 [Josh Rosen] Remove accidental duplicated code
07ff9ef [Josh Rosen] Basic support for large rows in BytesToBytesMap.

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