Added batch write buffer

[mtmk]

• Changed publish to be async
• Added BatchBuffer to improve write performance

Tests below were run against nats bench:

nats bench x --sub 1 --msgs 1000000

nats bench

nats bench x --pub 1 --msgs 1000000

Sub stats: 1,457,860 msgs/sec ~ 177.96 MB/sec

with batch

swift run -c release bench pub x 1000000

Sub stats: 148,782 msgs/sec ~ 18.16 MB/sec

with batch disabled

swift run -c release -Xswiftc -DSWIFT_NATS_BATCH_BUFFER_DISABLED bench pub x 1000000

Sub stats: 18,998 msgs/sec ~ 2.32 MB/sec

[piotrpio]

LGTM!