os: make tmpdir be const value

[yorkie]

This patch also make the function getTmpdir() be called once with a little improvement, but the main purpose of this PR is to make tmpdir as const as well.

[brendanashworth]

Couldn't os.tmpdir() change during program execution if the user changes one of the environment variables?

[jbergstroem]

@brendanashworth I recall python's tempfile module using the same assumption (read env on every call). I think we should avoid making this a const.

[silverwind]

With environment variables read only once at startup, it should currently behaves as a constant, unless the user changes the variables during runtime (unlikely), which we would take away the ability to do so.

[ChALkeR]

@silverwind Could a variable in process.env be changed from outside of the js code while the iojs process is running?

[targos]

@ChALkeR yes it can. process.env[name] calls getenv.
But as @silverwind said it is very unlikely that the environment variables will change for the running process.

[domenic]

It is a function, after all: os.tmpdir(), not os.tmpdir. As such I think it's pretty reasonable to expect it will do some work every time you call it, and be always up to date. If people want a const value they can just do const t = os.tmpdir().

[silverwind]

The process gets a snapshot of the environment on startup and the only way for it to change is through assignment to process.env.TMP and similar variables from within the process. This PR would take that option away from the user.

[ChALkeR]

@silverwind If that is true, why not take a snapshot `process.env` as a simple js object at startup? Atm, `process.env.foo` is much slower than it would be if that was converted to an object. But can't some native code call `setenv("foo", "bar")` and expect `process.env.foo` to be changed on the js side?

[vkurchatkin]

-1. In most usage patterns it is cached anyway

[Fishrock123]

Closing for now, seems like the consensus is this isn't too great, and you can cache it yourself where applicable in programs. :)