Update #5100
Update #6086

Remove switchReader, switchWriter, switchReaderPair,
switchWriterPair, etc.

Now it only maintains pools of bufio Readers and Writers, but
uses Reset instead of working around all their
previously-associated state.

Compared to before the bufio Reset change, it's the same number of
allocations, and also faster:

benchmark                                   old ns/op    new ns/op    delta
BenchmarkClientServer                          111218       109828   -1.25%
BenchmarkClientServerParallel4                  70580        70013   -0.80%
BenchmarkClientServerParallel64                 72636        68919   -5.12%
BenchmarkServer                                139858       137068   -1.99%
BenchmarkServerFakeConnNoKeepAlive              14619        14314   -2.09%
BenchmarkServerFakeConnWithKeepAlive            12390        11361   -8.31%
BenchmarkServerFakeConnWithKeepAliveLite         7630         7306   -4.25%
BenchmarkServerHandlerTypeLen                    9688         9342   -3.57%
BenchmarkServerHandlerNoLen                      8700         8470   -2.64%
BenchmarkServerHandlerNoType                     9255         8949   -3.31%
BenchmarkServerHandlerNoHeader                   7058         6806   -3.57%

benchmark                                  old allocs   new allocs    delta
BenchmarkClientServer                              61           61    0.00%
BenchmarkClientServerParallel4                     61           61    0.00%
BenchmarkClientServerParallel64                    61           61    0.00%
BenchmarkServer                                    16           16    0.00%
BenchmarkServerFakeConnNoKeepAlive                 24           24    0.00%
BenchmarkServerFakeConnWithKeepAlive               19           19    0.00%
BenchmarkServerFakeConnWithKeepAliveLite            9            9    0.00%
BenchmarkServerHandlerTypeLen                      17           17    0.00%
BenchmarkServerHandlerNoLen                        14           14    0.00%
BenchmarkServerHandlerNoType                       15           15    0.00%
BenchmarkServerHandlerNoHeader                      9            9    0.00%

benchmark                                   old bytes    new bytes    delta
BenchmarkClientServer                            6988         6985   -0.04%
BenchmarkClientServerParallel4                   6979         6985    0.09%
BenchmarkClientServerParallel64                  7002         7019    0.24%
BenchmarkServer                                  1846         1848    0.11%
BenchmarkServerFakeConnNoKeepAlive               2420         2412   -0.33%
BenchmarkServerFakeConnWithKeepAlive             2126         2129    0.14%
BenchmarkServerFakeConnWithKeepAliveLite          989          990    0.10%
BenchmarkServerHandlerTypeLen                    1818         1819    0.06%
BenchmarkServerHandlerNoLen                      1775         1777    0.11%
BenchmarkServerHandlerNoType                     1783         1785    0.11%
BenchmarkServerHandlerNoHeader                    989          990    0.10%

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12708046