Avoid exception if fail to insert block to block-cache

### Expected behavior
If fail to insert block to block-cache, just abot the insert but return the result as normal.

### Actual behavior
If we use default `ReadOptions` with `strict_capacity_limit` block cache, RocksDB would throw exception if `Insert failed due to LRU cache being full`.

### Steps to reproduce the behavior
This is because current `RetrieveBlock` would [check the status of `MaybeReadBlockAndLoadToCache`](https://github.com/facebook/rocksdb/blob/add68bd28a512da751e2bdc612685fdeb7e6dde4/table/block_based/block_based_table_reader.cc#L1909-L1916):

~~~c
s = MaybeReadBlockAndLoadToCache(
prefetch_buffer, ro, handle, uncompression_dict, wait_for_cache,
block_entry, block_type, get_context, lookup_context,
/*contents=*/nullptr);

if (!s.ok()) {
return s;
}
~~~

And if fail to insert, the block cache would report [incomplete status](https://github.com/facebook/rocksdb/blob/add68bd28a512da751e2bdc612685fdeb7e6dde4/cache/lru_cache.cc#L326-L339):

~~~c
if ((usage_ + total_charge) > capacity_ &&
(strict_capacity_limit_ || handle == nullptr)) {
e->SetInCache(false);
if (handle == nullptr) {
// Don't insert the entry but still return ok, as if the entry inserted
// into cache and get evicted immediately.
last_reference_list.push_back(e);
} else {
if (free_handle_on_fail) {
delete[] reinterpret_cast(e);
*handle = nullptr;
}
s = Status::Incomplete("Insert failed due to LRU cache being full.");
}
~~~

As more and more applications move on cloud, and fine-granularity memory control is required in many use cases. I think enabling the strict capacity limit of block cache in production environment should be something valueable.
How about this solution:
1. Introduce a new status as `FailInsertCache`.
1. Introduce a new filed named `fill_cache_if_possible` in ReadOptions, which means if cannot fill block to cache, we would not treat `FailInsertCache` status as a problem, and continue the read process as normal.

What do you think of this problem and the propsed solution?