Refactor collection_jitter and flush_jitter

use a common function between collection_jitter and flush_jitter. which
creates the same behavior between the two options.

going forward, both jitters will be random sleeps that get re-evaluated
at runtime for every interval (previously only collection_jitter did
this)

also fixes behavior so that both jitters will exit in the event of a
process exit.

closes #1296

internal/config/config.go

@@ -58,7 +58,6 @@ func NewConfig() *Config {
 			Interval:      internal.Duration{Duration: 10 * time.Second},
 			RoundInterval: true,
 			FlushInterval: internal.Duration{Duration: 10 * time.Second},
-			FlushJitter:   internal.Duration{Duration: 5 * time.Second},
 		},
 
 		Tags:          make(map[string]string),