Prior to this patch Tarantool GDB-extension failed during loading if
`main_cord` symbol is optimized out. As a result fiber-related commands
were not available, namely `info tt-fibers` and `tt-fiber`.

Closes #1206

@TarantoolBot document
Title: Adjusted Tarantool GDB-extension to avoid load failure.

Implementation of cartridge bootstrap in `tt` needs to know aliases of
all instances to work properly. Aliases of instances are managed with
`membership` module which spreads out certain instance alias across the
rest of instances using `gossip` protocol. At the very start of
an instance only that instance "knows" its alias, others get this
knowledge with some delay. That means in turn that the topology obtained
from the different instances might differ in terms of aliases.

Prior to this patch topology from a single instance was used and if at
that moment this instance haven't collected information about all the
aliases, bootstrap failed. Now if any alias is missing we are trying to
get it from the topologies obtained from the other instances until all
aliases are resolved.

The patch fixes flaky test so there is not need to introduce new tests.

Closes #TNTP-3709