Uniswap registry fetch using all pairs by default

[diwu1989]

Uniswap registry was using pairwise combination to fetch pairs by default:
if (!this.opts?.fetchUsingAllPairs), fetchUsingAllPairs === undefined and negated become true

Given the number of tokens in the default token whitelist is pretty large now, this causes over 2k calls into the factory.

The actual number of pairs created by the factories are a lot smaller:

ubeswap nPairs 548
sushiswap nPairs 146
celodex nPairs 28

We can save the calls if we default fetchUsingAllPairs to true

[diwu1989]

unless fetchUsingAllPairs is not undefined and is explicitly set to false
we will use using the allPairsLength and allPairs factory methods

[zviadm]

I just changed name of the flag and made it default to false

[diwu1989]

I see, i like the new var name better