Revisit expensive locking in TXManager and TXModifications

[mrks]

Currently, we acquire a spinlock whenever we insert into the list of data inserts and deletes. This is because there might be multiple change operations running concurrently within the same transaction. Thus, we need to allow thread-safe operations on both the map<table, pos_list> and the pos_lists themselves.

Acquiring this lock is potentially expensive - thus we should try and find a better way that still guarantees thread safety.