Previously, RBF shared a list of WAL segments between the DB & Tx.
However, this increased the need for mutexes to access the data.
WAL segments are effectively immutable on-disk so the list of segments
has been refactored so that changes to the segment list are done via
copy-on-write which allows read transactions to access segment data
without a mutex.
The database checkpointing can remove early, unused segments and
there is an update/add check to make sure that Tx segments pushed back
to the DB do not include removed segments.
This commit adds the ability to start a transaction with an exclusive
lock for the entire database. This ensures no other read or write
transactions can run at the same time. Writes in this mode write
directly to the database and skip the WAL entirely.
