Reduce memory usage of capture plugin

[nicoddemus]

From #3323 (comment):

while pytests also keeps full reports, i beleive we should ... have items and reports drop more details/logging data in some way thats accessible/reloadable

[pytestbot]

GitMate.io thinks possibly related issues are #402 (let the capture plugin provide a way to run code with capture suspended), #1585 (Capture is all wrong), #3328 (Reduce memory usage of junitxml), #1521 (High memory usage if tests called by test case name), and #2968 (Memory usage in 3.3.0).