function mergeab(ab1, ab2) {
        var tmp = new Uint8Array(ab1.byteLength + ab2.byteLength);
        tmp.set(new Uint8Array(ab1), 0);
        tmp.set(new Uint8Array(ab2), ab1.byteLength);
        return tmp.buffer;
    }
Merge 2 ArrayBuffers
This is a convenience function to assist user receiver functions that
want to aggregate multiple payloads.
@function mergeab
@memberof Quiet
@param {ArrayBuffer} ab1 - beginning ArrayBuffer
@param {ArrayBuffer} ab2 - ending ArrayBuffer
@returns {ArrayBuffer} buf - ab1 merged with ab2