significant memory consuming during cluster migration

[BorysTheDev]

given:

two migration processes from one node with a replica to another 2 nodes
the size of the data 2GB
migration for all slots
before migration source node memory consumption is 2.2GB
problem: during migration memory consumption sometimes increases up to 3.6 GB (not every time, also have seen 2.8GB and 2.4GB). Without replication, the maximum memory consumption for this scenario I've seen is 2.8GB.

We must investigate why we have such memory consumption during migration and reduce it. Reproducibility rate 50%.
#3939 reproduce the problem

[BorysTheDev]

During migration we do next steps:

RestoreStreamer::WriteEntry() where we serialize the object and create a string with data
JournalWriter::Write() where we do copy of the string
JournalStreamer::Write() where we do one more copy and caching data for sending
If the serialized object is quite big and we send data not fast the size of JournalStreamer::pending_buf_ can be significantrly increased