XL/PutObject: Add single putObject and multipart caching. (#2115)

- Additionally adds test cases as well for object cache.
- Adds auto-expiry with expiration and cleanup time interval.

Fixes #2080
Fixes #2091

pkg/objcache/README.md

@@ -0,0 +1,62 @@
+```
+PACKAGE DOCUMENTATION
+
+package objcache
+    import "github.com/minio/minio/pkg/objcache"
+
+    Package objcache implements in memory caching methods.
+
+VARIABLES
+
+var DefaultExpiry = time.Duration(72 * time.Hour) // 72hrs.
+
+    DefaultExpiry represents default time duration value when individual
+    entries will be expired.
+
+var ErrCacheFull = errors.New("Not enough space in cache")
+    ErrCacheFull - cache is full.
+
+var ErrKeyNotFoundInCache = errors.New("Key not found in cache")
+    ErrKeyNotFoundInCache - key not found in cache.
+
+var NoExpiry = time.Duration(0)
+    NoExpiry represents caches to be permanent and can only be deleted.
+
+TYPES
+
+type Cache struct {
+
+    // OnEviction - callback function for eviction
+    OnEviction func(key string)
+    // contains filtered or unexported fields
+}
+    Cache holds the required variables to compose an in memory cache system
+    which also provides expiring key mechanism and also maxSize.
+
+func New(maxSize uint64, expiry time.Duration) *Cache
+    New - Return a new cache with a given default expiry duration. If the
+    expiry duration is less than one (or NoExpiry), the items in the cache
+    never expire (by default), and must be deleted manually.
+
+func (c *Cache) Create(key string, size int64) (w io.WriteCloser, err error)
+    Create - validates if object size fits with in cache size limit and
+    returns a io.WriteCloser to which object contents can be written and
+    finally Close()'d. During Close() we checks if the amount of data
+    written is equal to the size of the object, in which case it saves the
+    contents to object cache.
+
+func (c *Cache) Delete(key string)
+    Delete - delete deletes an entry from the cache.
+
+func (c *Cache) DeleteExpired()
+    DeleteExpired - deletes all the expired entries from the cache.
+
+func (c *Cache) Open(key string) (io.ReadSeeker, error)
+    Open - open the in-memory file, returns an in memory read seeker.
+    returns an error ErrNotFoundInCache, if the key does not exist.
+
+func (c *Cache) StopExpiry()
+    StopExpiry sends a message to the expiry routine to stop expiring cached
+    entries. NOTE: once this is called, cached entries will not be expired
+    if the consume has called this.
+```