ref: Remove keepalive:true as a default and document payload size

[kamilogorek]

Fixes: #1470
Fixes: #1464
Fixes: #339

Re: getsentry/sentry-docs#310

[HazAT]

For new SDKs it's here: #1497

[HazAT]

LGTM, btw docs are gone, you need to change it in the docs repo.
Also we should not forget when we release raven-js we need to update bower also.

[codecov-io]

Codecov Report

Merging #1496 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1496   +/-   ##
=======================================
  Coverage   72.79%   72.79%           
=======================================
  Files          42       42           
  Lines        1283     1283           
  Branches      242      242           
=======================================
  Hits          934      934           
  Misses        341      341           
  Partials        8        8

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