Fixes #3593: btl_openib: change SRQ defaults

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r28440 [[BR]]
* btl_openib: changed default SRQ settings:
    * increase number of wqe to minimize number of RNRs
    * it is better to have high watermark and post relatively small number of wqes
    * increased TX queue size


r28441 [[BR]]
* correction of r28440
Add bullet about openib SRQ parameters.