[SPARK-10984] Simplify *MemoryManager class structure

This patch refactors the MemoryManager class structure. After #9000, Spark had the following classes:

- MemoryManager
- StaticMemoryManager
- ExecutorMemoryManager
- TaskMemoryManager
- ShuffleMemoryManager

This is fairly confusing. To simplify things, this patch consolidates several of these classes:

- ShuffleMemoryManager and ExecutorMemoryManager were merged into MemoryManager.
- TaskMemoryManager is moved into Spark Core.

**Key changes and tasks**:

- [x] Merge ExecutorMemoryManager into MemoryManager.
  - [x] Move pooling logic into Allocator.
- [x] Move TaskMemoryManager from `spark-unsafe` to `spark-core`.
- [x] Refactor the existing Tungsten TaskMemoryManager interactions so Tungsten code use only this and not both this and ShuffleMemoryManager.
- [x] Refactor non-Tungsten code to use the TaskMemoryManager instead of ShuffleMemoryManager.
- [x] Merge ShuffleMemoryManager into MemoryManager.
  - [x] Move code
  - [x] ~~Simplify 1/n calculation.~~ **Will defer to followup, since this needs more work.**
- [x] Port ShuffleMemoryManagerSuite tests.
- [x] Move classes from `unsafe` package to `memory` package.
- [ ] Figure out how to handle the hacky use of the memory managers in HashedRelation's broadcast variable construction.
- [x] Test porting and cleanup: several tests relied on mock functionality (such as `TestShuffleMemoryManager.markAsOutOfMemory`) which has been changed or broken during the memory manager consolidation
  - [x] AbstractBytesToBytesMapSuite
  - [x] UnsafeExternalSorterSuite
  - [x] UnsafeFixedWidthAggregationMapSuite
  - [x] UnsafeKVExternalSorterSuite

**Compatiblity notes**:

- This patch introduces breaking changes in `ExternalAppendOnlyMap`, which is marked as `DevloperAPI` (likely for legacy reasons): this class now cannot be used outside of a task.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9127 from JoshRosen/SPARK-10984.