Summary:
Stacked on https://github.com/facebook/rocksdb/issues/11572
* Minimize use of std::function and lambdas to minimize chances of
compiler heap-allocating closures (unnecessary stress on allocator). It
appears that converting FindSlot to a template enables inlining the
lambda parameters, avoiding heap allocations.
* Clean up some logic with FindSlot (FIXMEs from https://github.com/facebook/rocksdb/issues/11572)
* Fix handling of rare case of probing all slots, with new unit test.
(Previously Insert would not roll back displacements in that case, which
would kill performance if it were to happen.)
* Add an -early_exit option to cache_bench for gathering memory stats
before deallocation.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11601

Test Plan:
unit test added for probing all slots

## Seeing heap allocations
Run `MALLOC_CONF="stats_print:true" ./cache_bench -cache_type=hyper_clock_cache`
before https://github.com/facebook/rocksdb/issues/11572 vs. after this change. Before, we see this in the
interesting bin statistics:

```
size  nrequests
----  ---------
  32     578460
  64      24340
8192     578460
```
And after:
```
size  nrequests
----  ---------
  32  (insignificant)
  64      24370
8192     579130
```

## Performance test
Build with `make USE_CLANG=1 PORTABLE=0 DEBUG_LEVEL=0 -j32 cache_bench`

Run `./cache_bench -cache_type=hyper_clock_cache -ops_per_thread=5000000`
in before and after configurations, simultaneously:

```
Before: Complete in 33.244 s; Rough parallel ops/sec = 2406442
After:  Complete in 32.773 s; Rough parallel ops/sec = 2441019
```

Reviewed By: jowlyzhang

Differential Revision: D47375092

Pulled By: pdillinger

fbshipit-source-id: 46f0f57257ddb374290a0a38c651764ea60ba410