Added context.Context to public methods

[thrawn01]

Purpose

All public methods which make network calls or await on flush should accept a context.Context which allows the caller to cancel the call if needed.

Implementation

• Added context.Context to Put()
• Added context.Context to PutWithOptions()
• Added context.Context to Close()
• Added context.Context to FlushWAL()
• Added context.Context to TableStore methods
• Modified the ReadOnlyBlob interface to accept context.Context

NOTE: Adding context to Close allows the caller to add a timeout to how long the caller is willing to wait for the database to flush all it's buffers and close the database safely.