stats: Histogram recordValue() can cause infinite stack loop

**Histogram recordValue() can cause infinite stack loop**

The current implementation of a Histogram differs from the Gauge/Counter in that it is immediately delivered to the Statsd sink instead of being buffered and periodically flushed. This poses a problem in a scenario where `Histogram::recordValue` is called in the code path of the TCP stats flusher (such as the load balancer).

This issue was discovered during the hotrestart_test for #5234, where a value is recorded into a histogram inside of `LoadBalancerBase::choosePriority()`. Recording the histogram value resulted in an infinite recursion that looked something like this (top to bottom):

```
Envoy::Upstream::LoadBalancerBase::choosePriority()
Envoy::Stats::ParentHistogramImpl::recordValue()
Envoy::Stats::ThreadLocalStoreImpl::deliverHistogramToSinks()
Envoy::Stats::ThreadLocalStoreImpl::ScopeImpl::deliverHistogramToSinks()
Envoy::Extensions::StatSinks::Common::Statsd::TcpStatsdSink::onHistogramComplete()
Envoy::Extensions::StatSinks::Common::Statsd::TcpStatsdSink::TlsSink::onTimespanComplete()
Envoy::Extensions::StatSinks::Common::Statsd::TcpStatsdSink::TlsSink::write()
Envoy::Upstream::ClusterManagerImpl::tcpConnForCluster()
Envoy::Upstream::LoadBalancerBase::chooseHost()
Envoy::Upstream::EdfLoadBalancerBase::chooseHostOnce()
Envoy::Upstream::ZoneAwareLoadBalancerBase::hostSourceToUse()
Envoy::Upstream::LoadBalancerBase::chooseHostSet()
Envoy::Upstream::LoadBalancerBase::choosePriority() <--- back to start of loop
Envoy::Stats::ParentHistogramImpl::recordValue()
...
```

**Possible Solutions:**

1) *Buffer histogram stats for periodic flushing*

We can have a per-TlsSink vector of a struct containing the information passed to `TcpStatsdSink::TlsSink::onTimespanComplete`. So, a string and a chrono::milliseconds value. Instead of instantly calling `TlsSink::write`, we can push into the vector and schedule a timer for immediate callback. The timer can flush all pending timespans. @mattklein123 suggests that the vector will stabilize in size and optimizations can be made later.

2) *Mimic Gauge and Counter behavior*

Similar to what we see in `TcpStatsdSink::flush`.

3) *Passing in a flag to prevent recursive histogram calls*

Passing a flag around to prevent recursive calls to `Histogram::recordValue` would work, but it's inelegant and requires developers to be aware of this scenario when adding future instrumentation. I'm just adding this as an option that was considered, but dismissed.