The maximum map count boostrap check can be a hindrance to users that do
not own the underlying platform on which they are executing
Elasticsearch. This is because addressing it requires tuning the kernel
and a platform provider might now allow this, especially on shared
infrastructure. However, this bootstrap check is not needed if mmapfs is
not in use. Today we do not have a way for the user to communicate that
they are not going to use mmapfs. This commit therefore adds a setting
that enables the user to disallow mmapfs. When mmapfs is disallowed, the
maximum map count bootstrap check is not enforced. Additionally, we
fallback to a different default index store and prevent the explicit use
of mmapfs for an index.
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