Introduce discovery cache timeout values

[mateiidavid]

The proxy caches results in-memory, both for inbound and outbound service (and policy) discovery. While the proxy's default values are great in most cases, certain client configurations may require overrides. The proxy supports overriding the default values, however, it currently does not offer an easy way for users to configure them.

This PR introduces two new values in Linkerd's control plane chart. The values control the inbound and outbound cache discovery idle timeout -- the amount of time a result will be kept in the cache if unused. Setting this value will change the configuration for all injected proxies, but not for the control plane.

---

Tests

Overriding options through CLI

$ bin/linkerd install --set proxy.outboundDiscoveryCacheUnusedTimeout="10s" \
       --set proxy.inboundDiscoveryCacheUnusedTimeout="120s" \
       | k apply -f -

Control plane does not receive the overrides:

$ k get po -n linkerd linkerd-destination-77b4f9849b-ptlhk -o yaml | grep 'LINKERD2_PROXY_\\w*_DISCOVERY_IDLE_TIMEOUT' -A 1
<nothing>

but injected proxies do

$ kubectl apply -f curl.yaml
$ k get po curl-c48c64d4-26wtn -o yaml | grep 'LINKERD2_PROXY_\\w*_DISCOVERY_IDLE_TIMEOUT' -A 1
    - name: LINKERD2_PROXY_OUTBOUND_DISCOVERY_IDLE_TIMEOUT
      value: 10s
    - name: LINKERD2_PROXY_INBOUND_DISCOVERY_IDLE_TIMEOUT
      value: 120s

Overriding through Helm

$ helm dep update linkerd-control-plane
$ helm install linkerd2 \
       --set proxy.outboundDiscoveryCacheUnusedTimeout="30s" \
       --set proxy.inboundDiscoveryCacheUnusedTimeout="99s" \
       --set-file identityTrustAnchorsPEM=ca.crt \
       --set-file identity.issuer.tls.crtPEM=issuer.crt \
       --set-file identity.issuer.tls.keyPEM=issuer.key \
       --create-namespace \
       --namespace linkerd \
       --set linkerdVersion=dev-33adae2b-matei \ # needs to be overridden
       ./linkerd-control-plane/

Control plane will not receive overrides:

$ k get po -n linkerd linkerd-destination-66d7ffc5f9-m7l7z -o yaml | grep 'LINKERD2_PROXY_\\w*_DISCOVERY_IDLE_TIMEOUT' -A 1
<nothing>

$  k get po curl-c48c64d4-gbtjc -o yaml | grep 'LINKERD2_PROXY_\\w*_DISCOVERY_IDLE_TIMEOUT' -A 1
    - name: LINKERD2_PROXY_OUTBOUND_DISCOVERY_IDLE_TIMEOUT
      value: 30s
    - name: LINKERD2_PROXY_INBOUND_DISCOVERY_IDLE_TIMEOUT
      value: 99s

[adleong]

Probably don't need to describe the default value in the comment since we can see the default value in the second column now.