[18.09 backport] awslogs: account for UTF-8 normalization in limits

[thaJeztah]

backport of moby#37986 for 18.09
fixes moby#37747 for 18.09

git checkout -b 18.09_backport_moby_37747 ce-engine/18.09
git cherry-pick -s -S -x 1e8ef386279e2e28aff199047e798fad660efbdd
git push -u origin

cherry-pick was clean; no conflicts

The CloudWatch Logs API defines its limits in terms of bytes, but its
inputs in terms of UTF-8 encoded strings. Byte-sequences which are not
valid UTF-8 encodings are normalized to the Unicode replacement
character U+FFFD, which is a 3-byte sequence in UTF-8. This replacement
can cause the input to grow, exceeding the API limit and causing failed
API calls.

This commit adds logic for counting the effective byte length after
normalization and splitting input without splitting valid UTF-8
byte-sequences into two invalid byte-sequences.

Fixes moby#37747

[thaJeztah]

Failure on Janky is unrelated; it's caused by the swagger-generation, and could be fixed by moby#36714 but that needs a carry

12:23:26 The result of hack/generate-swagger-api.sh differs
12:23:26 
12:23:26 diff --git a/api/types/container/container_wait.go b/api/types/container/container_wait.go
12:23:26 index 9e3910a6b4..06b0f02077 100644
12:23:26 --- a/api/types/container/container_wait.go
12:23:26 +++ b/api/types/container/container_wait.go
12:23:26 @@ -7,14 +7,6 @@ package container

[andrewhsu]

LGTM