add SW_AWSLAMBDA_FLUSH, fix husky again

[tom-pytel]

Two things here:

• Added SW_AWSLAMBDA_FLUSH env var to explicitly enable / disable data flush on AWS Lambda function exit. This is meant for higher throughput functions where flushing after every call could be expensive. Turning off flush may lose spans but should be much more performant. Spans will simply be sent on subsequent invocations of the handler after the process wakes up.
• Fixed another problem with husky. Started getting the following error on clean install from scoped repo:

npm ERR! command sh -c husky install
npm ERR! .git can't be found (see https://git.io/Jc3F9)

Seems to be typicode/husky#851 which is supposed to be fixed. Someone who understands this piece of software should double check that it is integrated correctly because this is the second problem encountered in just a couple of days.