Load generator: add a new mode to pre-generate load offline

[marta-lokhova]

Resolves stellar/stellar-core-internal#345
Resolves https://github.com/stellar/stellar-core-internal/issues/339

Offline load generation has a few nice properties:

• It eliminates account creation phase in our simulations completely. We can now pre-fill the DB by configuring GENESIS_TEST_ACCOUNT_COUNT. On my machine, creating 1,000,000 accounts takes ~20 seconds. This should speed up our iteration speed when working with simulations.
• It eliminates the overhead from creating transactions in real-time, and allows for a more realistic performance measurement. Transactions are pre-generated into LOADGEN_PREGENERATED_TRANSACTIONS_FILE via command line.
• It removes mempool race conditions that are making the simulation fail incorrectly (because we can create arbitrarily many accounts very quickly now, we can wait a long time before re-using the same account)

In addition, the new loadgen skips certain expensive validity checks, and adds pre-generated transactions directly into the tx queue, this further reduces the overhead of a load generation step.

[SirTyson]

Overall a very helpful change! A few comments from me, mostly minor stuff.

[SirTyson]

LGTM