Swap segment strategy

[MarinPostma]

This PR proposes a mecanism to configure when the current segment should be swapped with more control.

For this end, the SegmentSwapTrait is introduced that let's us define and compose strategies for when to swap the current segment

Two base strategies are also implemented:

• a frame count based strategy: swap is triggered if the number of frame in the wal exceeds some threshold
• a duration based strategy: swap is triggered if the last swap occured later that some duration

those strategy are then composed and used in SharedWal, with the following hardcoded setting:

• if the wal was last swapped more than 5 minutes ago, swap it
• if the wal contains more that 20K frames, swap it
• if the wal contains more than 1k frames and it is older than 30s, swap it.