Warn on startup if vm.max_map_count < 2 * max connections

In our production notes we recommend setting:

vm.max_map_count value of 128000

This corresponds to our default max connections of 64k, as each connection requires 2 maps.

Upon startup, the mongod should check this value (/proc/sys/vm/max_map_count) and warn if it doesn't equal 2*max connections.

Acceptance Criteria: We warn users that vm.max_map_count doesn't exceed 2x max connections