Max price

[Sriep]

A brief description of the changes in this PR:
The max price for new allocation request gets type cast several times. Using math.MaxInt64 can lead to problems.

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• 0chain: Replace add allocation with update 0chain#1424
• blobber:
• gosdk:
• system_test:
• zwalletcli:
• Other: ...

[service-0chain]

Manual system tests [failure] with the following config

config	value
tests skipped	FALSE
system-tests	master
gosdk	NOT OVERRIDDEN
zboxcli	max_prices
zwalletcli	staging
0chain	update_allocation
blobber	staging
authorizer	staging
0box	staging
0block	staging
0dns	staging
explorer	master
0proxy	staging
0search	staging

[service-0chain]

Manual system tests [success] with the following config

config	value
tests skipped	FALSE
system-tests	master
gosdk	NOT OVERRIDDEN
zboxcli	max_prices
zwalletcli	staging
0chain	update_allocation
blobber	staging
authorizer	staging
0box	staging
0block	staging
0dns	staging
explorer	master
0proxy	staging
0search	staging

[stewartie4]

makes sense LGTM