Fix Embag Memory Leaks

[liambenson]

Fixes: DATA-526

Description of Changes

There was a memory leak resulting from the recent performance changes due to circular shared_ptrs. This fixes the issue by storing weak_ptrs to the vector of RosValues and then having any interfaces that access the RosValues lock the weak_ptr and return a RosValuePointer which stores the locked weak_ptr (which is just a shared_ptr).

Test Plan

Tests + ensured that -fsanitize=address does not show any errors.

[suurjaak]

+1 to getting the leak fixed. At least on the Python side, the memory leak makes embag unusable for any larger bag.

E.g. just reading through a bag with thousands of messages will eat memory until it's all gone and system freezes:

import time, embag
b = embag.Bag("somebiggerfile.bag")
count = 0
for m in b.read_messages(list(b.topics())):
    count += 1
    if not count % 1000: print(count)
    time.sleep(0.0001)

Same for the results from embag.View, at least when using msg.data().