fix(lambda): make lambda active only after successful start

[NamanJain8]

There is a logical race condition that causes panic. This happens because the node process did not complete its initialization before being killed. This PR fixes that issue.

This PR also increases the default restart timeout from 10s to 30s to make it more generous.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xbf5f7b]

goroutine 175 [running]:
os.(*Process).signal(0x0, 0x279b908, 0x32592d8, 0x0, 0x0)
	/usr/local/go/src/os/exec_unix.go:63 +0x3b
os.(*Process).Signal(...)
	/usr/local/go/src/os/exec.go:135
os.(*Process).kill(...)
	/usr/local/go/src/os/exec_posix.go:66
os.(*Process).Kill(...)
	/usr/local/go/src/os/exec.go:120
github.com/dgraph-io/dgraph/dgraph/cmd/alpha.setupLambdaServer.func2(0xc0003be1c0)
	/data/go/src/github.com/dgraph-io/dgraph/dgraph/cmd/alpha/run.go:556 +0x191
github.com/dgraph-io/dgraph/dgraph/cmd/alpha.setupLambdaServer.func3(0xc000118ae0, 0xc0005020a8, 0xc0000df6b0)
	/data/go/src/github.com/dgraph-io/dgraph/dgraph/cmd/alpha/run.go:577 +0x91
created by github.com/dgraph-io/dgraph/dgraph/cmd/alpha.setupLambdaServer
	/data/go/src/github.com/dgraph-io/dgraph/dgraph/cmd/alpha/run.go:568 +0x82b

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This change is