[FEA] Support unspill for SpillableHostBuffer

[binmahone]

Is your feature request related to a problem? Please describe.

Currently once a SpillableHostBuffer is spilled from memory to disk, all subsequent invocations of SpillableHostBuffer#getHostBuffer will read and deserialize from disk. It's very costly and won't be acceptable in cases where we will call the getHostBuffer multiple times.

One example would be the Kudo shuffle read concat case, let's asssume the read KudoTables are placed into a spillable state (by wrapping the HostMemoryBuffer in SpillableHostBuffer), then when doing the kudo concat, we will have to frequently and randomly call SpillableHostBuffer#getHostBuffer, since we know kudo concat adopts a random read visitor to read all the input KudoTables. It's a performance nightmere if we have to read from disk every time.