fix: Tree filled lower than order is not searchable

[anuragvohraec]

Issue being fixed or implemented

As of now if the tree is filled less then order size, one cannot search/delete/replace documents if using $lte to query. Example code:

const {SBTree} = require('sbtree');
const tree = new SBTree({order: 3});

(async()=>{
    await tree.insertDocuments({
        doc_id: '754aeea7c0919797d6eb2710d200eafd',
        key: 'VATEmq',
        value: 4,
        ts: 1588913801357
    })
    const t1 = new Date().getTime()-10000;
    console.log(t1, typeof t1);
    console.log(1588913801357<t1);
    console.log(await tree.deleteDocuments({ts: {$lte: t1}}))
})().then(v=>{
    console.log("Complete");
})

In above code, we create a tree with order size three and filled it with only one elements.
Now if we delete the document using query `{ts: {$lte:t1}} , the single documents though valid for deletio is not found out and deleted/

What was done

The problem seems to be occuring due to leafIndex incrementation below order level.
Have applied a decrement operation to balance the mismatch.

How Has This Been Tested?

Have re-run the sample code, which failed.

Notes

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation