scheduler: only add allocs to network and device trackers if required

[nickethier]

There are two structs used in in scheduling to track usage of networks and devices. While they are only used if an alloc requires a network or device, they always are initialized regardless. In many cases this initialization overhead is minimal. However, in cases where the cardinality of allocs assigned to ranked nodes are high, memory and cpu cycles are wasted accounting the devices and networks used by these allocs if that accounting is never used to rank the node, e.g. the proposed allocation doesn't require a network or device.

This PR moves the initialization to a function that is called by sync.Once when needed. We do call netIdx.Release() always so I made sure that accepted a nil receiver without panicing.

[schmichael]

I like think the functionality is right, but let's rework this not require a closure or a sync.Once. Since netIdx is never shared between goroutines a bool or if netIdx == nil { init() } check is sufficient.

However, I want us to at least consider refactoring NetworkIndex instead before merging because like many of networking structs it's used for slightly different purposes in slightly different places.

Here NetworkIndex should lazily evaluate SetNode+AddAllocs only when AssignPorts is called.

The collision detection logic that requires immediate (non-lazy) evaluation is only used in the planner.

Perhaps SetNode+AddAllocs could be made lazy (just setters that copy the pointer they're passed; probably inlined by the compiler), and a new Check() (ok bool) method could be added?

At any rate, this is great work and the refactorings may not pan out, but I think pushing a bit further on it could make it more readable and perhaps even a teeny tiny bit more efficient? That may not be true since I think in the no-networks path your approach will incur 0 heap allocations.

[hashicorp-cla]

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