FIFO cache policy Key Removal mismatches actual removed key about caches

[vpetrigo]

Hello @sg-james,

Thank you for the report and a PR! I will take a look at your changes.

[sg-james]

I managed to create a simple test case that catches the bug

TEST(FIFOCache, Remove_Test_FifoRemoveBug_issue21) {

    constexpr std::size_t TEST_SIZE = 3;
    fifo_cache_t <std::string, std::size_t> fc(TEST_SIZE);

    for (std::size_t i = 0; i < TEST_SIZE; ++i) {
        fc.Put(std::to_string(i), i);
    }
                                                          // CachePolicy { 2, 1, 0 }        CacheMap { 0, 1, 2 }
    const int specific_key = 1;
    EXPECT_TRUE(fc.Remove(std::to_string(specific_key))); // CachePolicy { 2, 1 } -> 0      CacheMap { 0, 2 }

    fc.Put(std::to_string(3), 3);                         // CachePolicy { 3, 2, 1 }        CacheMap { 0, 2, 3 }
    fc.Put(std::to_string(4), 4);                         //             { 4, 3, 2 } -> 1   CacheMap.erase("1") // error

    EXPECT_EQ(fc.Size(), TEST_SIZE);
}

My PRs approach trades complexity for memory, but perhaps an O(n) removal lookup would be preferable to a second data structure?