Bug 927705 (part 1) - Remove PL_DHashTableSetAlphaBounds() and the supporting machinery.  r=jorendorff.

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extra : rebase_source : 0b97b3276ec91d417fcfc12fe871d93b0cbd2263
Bug 927705 (part 2a) - Increase pldhash's max capacity from 1<<23 to 1<<26, and protect against uint32_t overflow when computing storage size.  r=jorendorff.

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extra : rebase_source : 9293ec75e5ba8ad416d66ad6bf0c0788c0d35b02
Bug 927705 (part 2b) - Never overload pldhash past 75% full, because performance plummets near the end.  r=jorendorff.

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extra : rebase_source : 7d3a0594e49ceb89550861debca03a40d3e479c1
Bug 927705 (part 3) - Add a C++ unit test for pldhash.  r=jorendorff.

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extra : rebase_source : 58cddfff7da6116af7512097c3a6172ca1c06b87