render_markdown_path: Use limited caching.
render_markdown_path: Use limited caching.

This results in an almost 50% increase in performance in the slow
test_urls test.

Fixes #6901.

hackerkid/zulip,hackerkid/zulip,tommyip/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,kou/zulip,rishig/zulip,timabbott/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,brainwane/zulip,synicalsyntax/zulip,synicalsyntax/zulip,dhcrzf/zulip,brainwane/zulip,synicalsyntax/zulip,rishig/zulip,dhcrzf/zulip,zulip/zulip,rishig/zulip,jackrzhang/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,tommyip/zulip,punchagan/zulip,dhcrzf/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,jackrzhang/zulip,eeshangarg/zulip,shubhamdhama/zulip,dhcrzf/zulip,rishig/zulip,eeshangarg/zulip,rht/zulip,showell/zulip,jackrzhang/zulip,kou/zulip,timabbott/zulip,kou/zulip,jackrzhang/zulip,rishig/zulip,andersk/zulip,jackrzhang/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,tommyip/zulip,punchagan/zulip,kou/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,brainwane/zulip,jackrzhang/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,shubhamdhama/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,dhcrzf/zulip,showell/zulip,jackrzhang/zulip,synicalsyntax/zulip,zulip/zulip,timabbott/zulip,dhcrzf/zulip,synicalsyntax/zulip,tommyip/zulip,brainwane/zulip,rishig/zulip,andersk/zulip,showell/zulip,kou/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,tommyip/zulip,showell/zulip,andersk/zulip,tommyip/zulip,rishig/zulip,shubhamdhama/zulip,punchagan/zulip,rht/zulip,andersk/zulip,shubhamdhama/zulip,timabbott/zulip,dhcrzf/zulip,kou/zulip,synicalsyntax/zulip,eeshangarg/zulip,tommyip/zulip,shubhamdhama/zulip,shubhamdhama/zulip,showell/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip