MultiplexedConnection: Remove unnecessary allocation in send.

[nihohit]

Instead of using a Vec and allocating / popping values from it,
single requests will now create only a single Value object, which will
reduce the heap usage for single requests.
This gives us a small but consistent perf improvement of about 1% for Node benchmarks, and ~5% for Rust benchmarks.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[nihohit]

<style> </style>

is_cluster	client	num_of_tasks	data_size	language	baseline	baseline-local	multiplex	ratio	multiplex-local	ratio
FALSE	babushka socket	10	100	node	44987	83463.91667	47740.81818	1.061213644	84161.83333	1.008361897
FALSE	babushka socket	100	100	node	125619.4545	155411.3333	127005.4545	1.011033323	156535.6667	1.007234565
FALSE	babushka socket	1000	100	node	168319.0909	168156.6667	170059.9091	1.010342369	168566.5833	1.002437707
FALSE	babushka	10	100	rust	53822.5	175588.5	52596	0.977212132	181523.9167	1.033802992
FALSE	babushka	100	100	rust	375428.25	553940.8333	399197.25	1.063311698	604828.75	1.091865256
FALSE	babushka	1000	100	rust	396085.75	456958.25	425199.75	1.073504285	491525.4167	1.075646225
TRUE	babushka socket	10	100	node	37278.09091		37748.63636	1.012622574
TRUE	babushka socket	100	100	node	118927.8182		118536	0.996705412
TRUE	babushka socket	1000	100	node	140978.4545		142286.5455	1.009278658
TRUE	babushka	10	100	rust	41588		42564.75	1.023486342
TRUE	babushka	100	100	rust	248708.5		256052.5	1.029528544
TRUE	babushka	1000	100	rust	271053.25		289565.5	1.068297466