internal/frontend: cache slow symbol searches for a long time
internal/frontend: cache slow symbol searches for a long time

If the request is for a symbol that takes a long time to search, cache
the results for a long time.

Change-Id: I68a901b81c854c30af01b626e00cf4d0de1ee59c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/356531
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite