Support memory cache

[wchaws]

Issue #, if available:

Fix: #167

Description of changes:

Support the following env to control cache behavior.

{
  "env": {
    "CACHE_TTL_SEC": "300",  // The cache ttl in seconds, Note that stale items are NOT preemptively removed by default. https://github.com/isaacs/node-lru-cache#ttl
    "CACHE_MAX_ITEMS": "10000", // The max cached keys count.
    "CACHE_MAX_SIZE_MB": "1024" // The max cached size in mb.
  }
}

If cache hit, the HTTP response will contain: X-Cached-Response: HIT

To check cached info: curl <endpoint>/debug and check logs(use CloudWatch filter expr { $.os.arch != "" }) in CloudWatch Logs. You will find something like below

{
    "os": {
        "arch": "x64",
        "cpus": 4,
        "loadavg": [
            12.23,
            9.71,
            7.15
        ]
    },
    "memory": {
        "stats": "free: 11.17 GB, total: 15.16 GB, usage 26.31 %",
        "free": 11995893760,
        "total": 16278593536,
        "usage": {
            "rss": 3009978368,
            "heapTotal": 43941888,
            "heapUsed": 21216176,
            "external": 747442337,
            "arrayBuffers": 502518701
        }
    },
    "resource": {
        "usage": {
            "userCPUTime": 10773049374,
            "systemCPUTime": 1577877281,
            "maxRSS": 4900488,
            "sharedMemorySize": 0,
            "unsharedDataSize": 0,
            "unsharedStackSize": 0,
            "minorPageFault": 501239508,
            "majorPageFault": 3166,
            "swappedOut": 0,
            "fsRead": 0,
            "fsWrite": 25328,
            "ipcSent": 0,
            "ipcReceived": 0,
            "signalsCount": 0,
            "voluntaryContextSwitches": 15725288,
            "involuntaryContextSwitches": 10475948
        }
    },
    "sharp": {
        "cache": {
            "memory": {
                "current": 1899,
                "high": 2346,
                "max": 2000
            },
            "files": {
                "current": 0,
                "max": 200
            },
            "items": {
                "current": 519,
                "max": 1000
            }
        },
        "simd": true,
        "counters": {
            "queue": 0,
            "process": 4
        },
        "concurrency": 4,
        "versions": {
            "aom": "3.5.0",
            "cairo": "1.17.6",
            "cgif": "0.3.0",
            "exif": "0.6.24",
            "expat": "2.5.0",
            "ffi": "3.4.4",
            "fontconfig": "2.14.1",
            "freetype": "2.12.1",
            "fribidi": "1.0.12",
            "gdkpixbuf": "2.42.10",
            "glib": "2.74.1",
            "gsf": "1.14.50",
            "harfbuzz": "5.3.1",
            "heif": "1.13.0",
            "imagequant": "2.4.1",
            "lcms": "2.14",
            "mozjpeg": "4.1.1",
            "orc": "0.4.33",
            "pango": "1.50.11",
            "pixman": "0.42.2",
            "png": "1.6.38",
            "proxy-libintl": "0.4",
            "svg": "2.55.1",
            "spng": "0.7.2",
            "tiff": "4.4.0",
            "vips": "8.13.3",
            "webp": "1.2.4",
            "xml": "2.10.3",
            "zlib-ng": "2.0.6"
        }
    },
    "lruCache": {  // HERE
        "keys": 10000,
        "sizeMB": 685.03,
        "ttlSec": 300
    }
}

Checklist

• 👋 I have run the unit tests, and all unit tests have passed.
• ⚠️ This pull request might incur a breaking change.

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