[exporter/splunk_hec] Apply compression if it's enabled in the config

[dmitryax]

The compression used to be enabled only if the payload size was greater than 1.5KB which significantly complicated the logic and made it hard to test. This change makes the compression unconditionally applied to the payload if it's enabled in the config. The benchmarking shows improvements in the throughput and CPU usage for large payloads and expected degradation for small payloads, which is acceptable given that it's not a common case.

Before:

Benchmark_pushLogData_compressed_10_10_1024
Benchmark_pushLogData_compressed_10_10_1024-10    	   10000	    105616 ns/op	   84893 B/op	    1285 allocs/op
Benchmark_pushLogData_compressed_10_10_8K
Benchmark_pushLogData_compressed_10_10_8K-10      	    4635	    260261 ns/op	  899893 B/op	    1130 allocs/op
Benchmark_pushLogData_compressed_10_10_2M
Benchmark_pushLogData_compressed_10_10_2M-10      	    3620	    334618 ns/op	 2993165 B/op	    1131 allocs/op
Benchmark_pushLogData_compressed_10_200_2M
Benchmark_pushLogData_compressed_10_200_2M-10     	     308	   3884943 ns/op	 4376605 B/op	   22031 allocs/op
Benchmark_pushLogData_compressed_100_200_2M
Benchmark_pushLogData_compressed_100_200_2M-10    	      32	  35486009 ns/op	17481330 B/op	  220032 allocs/op
Benchmark_pushLogData_compressed_100_200_5M
Benchmark_pushLogData_compressed_100_200_5M-10    	      31	  35574226 ns/op	20626891 B/op	  220032 allocs/op

After:

Benchmark_pushLogData_compressed_10_10_1024
Benchmark_pushLogData_compressed_10_10_1024-10    	    1850	    650097 ns/op	 3106085 B/op	    1192 allocs/op
Benchmark_pushLogData_compressed_10_10_8K
Benchmark_pushLogData_compressed_10_10_8K-10      	    7755	    158985 ns/op	   74715 B/op	    1109 allocs/op
Benchmark_pushLogData_compressed_10_10_2M
Benchmark_pushLogData_compressed_10_10_2M-10      	    7790	    152750 ns/op	   74079 B/op	    1109 allocs/op
Benchmark_pushLogData_compressed_10_200_2M
Benchmark_pushLogData_compressed_10_200_2M-10     	     332	   3607309 ns/op	 1459565 B/op	   22010 allocs/op
Benchmark_pushLogData_compressed_100_200_2M
Benchmark_pushLogData_compressed_100_200_2M-10    	      32	  34716503 ns/op	14595388 B/op	  220012 allocs/op
Benchmark_pushLogData_compressed_100_200_5M
Benchmark_pushLogData_compressed_100_200_5M-10    	      31	  34683440 ns/op	14596493 B/op	  220012 allocs/op