Update revid.go and config.go for new vring buffer

[Frank Cui]

In revid.go and config.go:

• Change imported package ring->vring.
• The RTMPRBSize and MTSRBSize arguments should be now referring to the maximum number of elements that can be stored in the buffer. Change variable names to RTMPMaxElements and MTSMaxElementsor something appropriate.
• The RTMPRBElementSize and MTSRBElementSize arguments should be now referring to the maximum capacity of the buffer in bytes. Change variable names to RTMPCapacity and MTSCapacity or something appropriate.
• Select reasonable default values for the above variables in config.go
• NewBuffer constructor now returns an additional error variable.

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[Saxon Milton]

Resolved in PR #324