Implement Fee Bumping

[Varunram]

Inline with #157, we can also implement fee bumping which ensures that transactions make it to the blockchain even if there is a sudden surge in fee paid. An alternate way is to wait until the surge ends but due to the non custodial model of the platform, a user may double spend those funds. As a result, marking all investment transactions as fee-bumpable both makes overall user experience better (you don't want to deny investments when the network is experiencing high throughput), and is the only viable option in the event of a network surge

More info: https://www.stellar.org/developers-blog/how-we-simplified-fees-on-the-stellar-network