Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

1 files changed, 4 insertions, 0 deletions

diff --git a/hugolib/site.go b/hugolib/site.go
index 2ffc3a346..0ca7a81b4 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -778,6 +778,10 @@ func (s *SiteInfo) DisqusShortname() string {
 	return s.Config().Services.Disqus.Shortname
 }
 
+func (s *SiteInfo) GetIdentity() identity.Identity {
+	return identity.KeyValueIdentity{Key: "site", Value: s.language.Lang}
+}
+
 // SiteSocial is a place to put social details on a site level. These are the
 // standard keys that themes will expect to have available, but can be
 // expanded to any others on a per site basis