collection.clear(pattern) corrupts entryCount by setting it to zero

[warner]

Describe the bug

@mhofman noticed that the clearInternal() function for (strong) virtual/durable collections always sets the |entryCount metadata field to 0, even if it was called with a pattern that prevents some keys from being deleted.

agoric-sdk/packages/swingset-liveslots/src/collectionManager.js

Line 589 in a7d06d8

syscall.vatstoreSet(prefix('|entryCount'), '0');

To Reproduce

const c = makeScalarBigMapStore();
c.init('a', 'value-a');
c.init('b', 'value-b');
c.clear(M.eq('a');

assert(c.has('a') === false); // ok
assert(c.has('b') === true); // ok
assert(c.getSize() === 1); // broken, returns 0

To Fix

We need to change clearInternal() to only set the count to zero in the clearInternalFull() branch, and decrement it by the number of deleted keys in the other branch.

The fix must also consider isDeleting, which is true when we're deleting the whole collection, which means we don't need to update the count (because we're going to delete the metadata too, once this function returns).