Add zstd compression support to SAPM client

[tigrannajaryan]

I added a simple benchmark of compression methods and a test to print compressed byte sizes. Take with a grain of salt since the input is based on just one trivial data generator and may not be representative of production loads.

For the inputs used zstd wins both in compression size and speed (more apparent in larger input sizes).

Compression Sizes

=== RUN   TestCompressionSize
Message byte size by batch and compression method.
Compression       none      gzip      zstd
Batch=1            148       139       135
Batch=10          1558       503       490
Batch=100        15958      2495      2360
Batch=1000      161590     22110     22978
Batch=10000    1644106    222217    181468
--- PASS: TestCompressionSize (0.13s)

Benchmark

BenchmarkCompression/none/batch=1-8                56662             85501 ns/op          233172 B/op         28 allocs/op
BenchmarkCompression/gzip/batch=1-8                27688             64055 ns/op          117560 B/op         35 allocs/op
BenchmarkCompression/zstd/batch=1-8                41965             69071 ns/op          174616 B/op         33 allocs/op
BenchmarkCompression/none/batch=100-8              20131             63991 ns/op          117124 B/op        325 allocs/op
BenchmarkCompression/gzip/batch=100-8               3043            408736 ns/op           54255 B/op        335 allocs/op
BenchmarkCompression/zstd/batch=100-8              10276            120161 ns/op           80178 B/op        330 allocs/op
BenchmarkCompression/none/batch=1000-8              3066            382949 ns/op          323067 B/op       3025 allocs/op
BenchmarkCompression/gzip/batch=1000-8               243           4787768 ns/op          379947 B/op       3038 allocs/op
BenchmarkCompression/zstd/batch=1000-8              1128           1014972 ns/op          391926 B/op       3034 allocs/op

[dmitryax]

LGTM